Publications by authors named "Lucio Isa"

84 Publications

Exploring the roles of roughness, friction and adhesion in discontinuous shear thickening by means of thermo-responsive particles.

Nat Commun 2021 Mar 5;12(1):1477. Epub 2021 Mar 5.

Laboratory for Soft Materials and Interfaces, Department of Materials, ETH Zurich, Zurich, Switzerland.

Dense suspensions of colloidal or granular particles can display pronounced non-Newtonian behaviour, such as discontinuous shear thickening and shear jamming. The essential contribution of particle surface roughness and adhesive forces confirms that stress-activated frictional contacts can play a key role in these phenomena. Here, by employing a system of microparticles coated by responsive polymers, we report experimental evidence that the relative contributions of friction, adhesion, and surface roughness can be tuned in situ as a function of temperature. Modifying temperature during shear therefore allows contact conditions to be regulated, and discontinuous shear thickening to be switched on and off on demand. The macroscopic rheological response follows the dictates of independent single-particle characterization of adhesive and tribological properties, obtained by colloidal-probe atomic force microscopy. Our findings identify additional routes for the design of smart non-Newtonian fluids and open a way to more directly connect experiments to computational models of sheared suspensions.
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http://dx.doi.org/10.1038/s41467-021-21580-yDOI Listing
March 2021

Sequential capillarity-assisted particle assembly in a microfluidic channel.

Lab Chip 2021 Jan 11. Epub 2021 Jan 11.

Institute of Environmental Engineering, Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, Stefano-Franscini-Platz 5, 8093 Zürich, Switzerland.

Colloidal patterning enables the placement of a wide range of materials into prescribed spatial arrangements, as required in a variety of applications, including micro- and nano-electronics, sensing, and plasmonics. Directed colloidal assembly methods, which exploit external forces to place particles with high yield and great accuracy, are particularly powerful. However, currently available techniques require specialized equipment, which limits their applicability. Here, we present a microfluidic platform to produce versatile colloidal patterns within a microchannel, based on sequential capillarity-assisted particle assembly (sCAPA). This new microfluidic technology exploits the capillary forces resulting from the controlled motion of an evaporating droplet inside a microfluidic channel to deposit individual particles in an array of traps microfabricated onto a substrate. Sequential depositions allow the generation of a desired spatial layout of colloidal particles of single or multiple types, dictated solely by the geometry of the traps and the filling sequence. We show that the platform can be used to create a variety of patterns and that the microfluidic channel easily allows surface functionalization of trapped particles. By enabling colloidal patterning to be carried out in a controlled environment, exploiting equipment routinely used in microfluidics, we demonstrate an easy-to-build platform that can be implemented in microfluidics labs.
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http://dx.doi.org/10.1039/d0lc00962hDOI Listing
January 2021

Near-zero surface pressure assembly of rectangular lattices of microgels at fluid interfaces for colloidal lithography.

Soft Matter 2021 Jan;17(2):335-340

Laboratory for Soft Materials and Interfaces, Swiss Federal Institute of Technology Zürich, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland.

Understanding and engineering the self-assembly of soft colloidal particles (microgels) at liquid-liquid interfaces is broadening their use in colloidal lithography. Here, we present a new route to assemble rectangular lattices of microgels at near zero surface pressure relying on the balance between attractive quadrupolar capillary interactions and steric repulsion among the particles at water/oil interfaces. These self-assembled rectangular lattices are obtained for a broad range of particles and, after deposition, can be used as lithography masks to obtain regular arrays of vertically aligned nanowires via wet and dry etching processes.
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http://dx.doi.org/10.1039/d0sm01823fDOI Listing
January 2021

Microscale Marangoni Surfers.

Phys Rev Lett 2020 Aug;125(9):098001

Laboratory for Soft Materials and Interfaces, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland.

We apply laser light to induce the asymmetric heating of Janus colloids adsorbed at water-oil interfaces and realize active micrometric "Marangoni surfers." The coupling of temperature and surfactant concentration gradients generates Marangoni stresses leading to self-propulsion. Particle velocities span 4 orders of magnitude, from microns/s to cm/s, depending on laser power and surfactant concentration. Experiments are rationalized by finite elements simulations, defining different propulsion regimes relative to the magnitude of the thermal and solutal Marangoni stress components.
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http://dx.doi.org/10.1103/PhysRevLett.125.098001DOI Listing
August 2020

Particle Surface Roughness as a Design Tool for Colloidal Systems.

Langmuir 2020 Sep 21;36(38):11171-11182. Epub 2020 Sep 21.

Department of Materials ETH Zurich, Laboratory for Soft Materials and Interfaces, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.

Control over the surface roughness of colloidal particles offers exciting opportunities to tailor the properties and the processing of a broad range of soft matter systems. Moreover, identifying surface roughness as a design parameter reveals the possibility to connect seemingly distinct phenomena and materials via the role played by roughness effects. In this feature article, we concisely review some approaches to synthesize and characterize rough colloidal particles, with a focus on model spherical colloids. We then discuss the impact that surface roughness has on both the high-shear rheology of dense suspensions and the stabilization of Pickering emulsions. Commenting on developments of our own research, we aim to offer an original perspective for a property-oriented development of colloidal particles that transcends classical divisions between materials and processes toward innovative solutions.
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http://dx.doi.org/10.1021/acs.langmuir.0c02050DOI Listing
September 2020

Feedback-controlled active brownian colloids with space-dependent rotational dynamics.

Nat Commun 2020 Aug 24;11(1):4223. Epub 2020 Aug 24.

Laboratory for Soft Materials and Interfaces, Department of Materials, ETH Zurich, 8093, Zurich, Switzerland.

The non-thermal nature of self-propelling colloids offers new insights into non-equilibrium physics. The central mathematical model to describe their trajectories is active Brownian motion, where a particle moves with a constant speed, while randomly changing direction due to rotational diffusion. While several feedback strategies exist to achieve position-dependent velocity, the possibility of spatial and temporal control over rotational diffusion, which is inherently dictated by thermal fluctuations, remains untapped. Here, we decouple rotational diffusion from thermal fluctuations. Using external magnetic fields and discrete-time feedback loops, we tune the rotational diffusivity of active colloids above and below its thermal value at will and explore a rich range of phenomena including anomalous diffusion, directed transport, and localization. These findings add a new dimension to the control of active matter, with implications for a broad range of disciplines, from optimal transport to smart materials.
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http://dx.doi.org/10.1038/s41467-020-17864-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7445303PMC
August 2020

Self-templating assembly of soft microparticles into complex tessellations.

Nature 2020 06 11;582(7811):219-224. Epub 2020 Jun 11.

Laboratory for Soft Materials and Interfaces, Department of Materials, ETH Zurich, Zurich, Switzerland.

Encoding Archimedean and non-regular tessellations in self-assembled colloidal crystals promises unprecedented structure-dependent properties for applications ranging from low-friction coatings to optoelectronic metamaterials. Yet, despite numerous computational studies predicting exotic structures even from simple interparticle interactions, the realization of complex non-hexagonal crystals remains experimentally challenging. Here we show that two hexagonally packed monolayers of identical spherical soft microparticles adsorbed at a liquid-liquid interface can assemble into a vast array of two-dimensional micropatterns, provided that they are immobilized onto a solid substrate one after the other. The first monolayer retains its lowest-energy hexagonal structure and acts as a template onto which the particles of the second monolayer are forced to rearrange. The frustration between the two lattices elicits symmetries that would not otherwise emerge if all the particles were assembled in a single step. Simply by varying the packing fraction of the two monolayers, we obtain not only low-coordinated structures such as rectangular and honeycomb lattices, but also rhomboidal, hexagonal and herringbone superlattices encoding non-regular tessellations. This is achieved without directional bonding, and the structures formed are equilibrium structures: molecular dynamics simulations show that these structures are thermodynamically stable and develop from short-range repulsive interactions, making them easy to predict, and thus suggesting avenues towards the rational design of complex micropatterns.
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http://dx.doi.org/10.1038/s41586-020-2341-6DOI Listing
June 2020

Active Brownian Motion with Orientation-Dependent Motility: Theory and Experiments.

Langmuir 2020 06 12;36(25):7066-7073. Epub 2020 Feb 12.

Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany.

Combining experiments on active colloids, whose propulsion velocity can be controlled via a feedback loop, and the theory of active Brownian motion, we explore the dynamics of an overdamped active particle with a motility that depends explicitly on the particle orientation. In this case, the active particle moves faster when oriented along one direction and slower when oriented along another, leading to anisotropic translational dynamics which is coupled to the particle's rotational diffusion. We propose a basic model of active Brownian motion for orientation-dependent motility. On the basis of this model, we obtain analytical results for the mean trajectories, averaged over the Brownian noise for various initial configurations, and for the mean-square displacements including their non-Gaussian behavior. The theoretical results are found to be in good agreement with the experimental data. Orientation-dependent motility is found to induce significant anisotropy in the particle displacement, mean-square displacement, and non-Gaussian parameter even in the long-time limit. Our findings establish a methodology for engineering complex anisotropic motilities of active Brownian particles, with a potential impact in the study of the swimming behavior of microorganisms subjected to anisotropic driving fields.
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http://dx.doi.org/10.1021/acs.langmuir.9b03617DOI Listing
June 2020

Poly--isopropylacrylamide Nanogels and Microgels at Fluid Interfaces.

Acc Chem Res 2020 Feb 15;53(2):414-424. Epub 2020 Jan 15.

Institute of Particle Technology (LFG) , Friedrich-Alexander University Erlangen-Nürnberg , Cauerstrasse 4 , 91058 Erlangen , Germany.

The confinement of colloidal particles at liquid interfaces offers many opportunities for materials design. Adsorption is driven by a reduction of the total free energy as the contact area between the two liquids is partially replaced by the particle. From an application point of view, particle-stabilized interfaces form emulsions and foams with superior stability. Liquid interfaces also effectively confine colloidal particles in two dimensions and therefore provide ideal model systems to fundamentally study particle interactions, dynamics, and self-assembly. With progress in the synthesis of nanomaterials, more and more complex and functional particles are available for such studies. In this Account, we focus on poly(-isopropylacrylamide) nanogels and microgels. These are cross-linked polymeric particles that swell and soften by uptaking large amounts of water. The incorporated water can be partially expelled, causing a volume phase transition into a collapsed state when the temperature is increased above approximately 32 °C. Soft microgels adsorbed to liquid interfaces significantly deform under the influence of interfacial tension and assume cross sections exceeding their bulk dimensions. In particular, a pronounced corona forms around the microgel core, consisting of dangling chains at the microgel periphery. These polymer chains expand at the interface and strongly affect the interparticle interactions. The particle deformability therefore leads to a significantly more complex interfacial phase behavior that provides a rich playground to explore structure formation processes. We first discuss the characteristic "fried-egg" or core-corona morphology of individual microgels adsorbed to a liquid interface and comment on the dependence of this interfacial morphology on their physicochemical properties. We introduce different theoretical models to describe their interfacial morphology. In a second part, we introduce how ensembles of microgels interact and self-assemble at liquid interfaces. The core-corona morphology and the possibility to force these elements into overlap upon compression results in a complex phase behavior with a phase transition between microgels with extended and collapsed coronae. We discuss the influence of the internal particle architecture, also including core-shell microgels with rigid cores, on the phase behavior. Finally, we present new routes for the realization of more complex structures, resulting from multiple deposition protocols and from engineering the interaction potential of the individual particles.
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http://dx.doi.org/10.1021/acs.accounts.9b00528DOI Listing
February 2020

Effect of the 3D Swelling of Microgels on Their 2D Phase Behavior at the Liquid-Liquid Interface.

Langmuir 2019 Dec 16;35(51):16780-16792. Epub 2019 Dec 16.

Institute of Physical Chemistry , RWTH Aachen University , Landoltweg 2 , 52056 Aachen , Germany.

We investigate soft, temperature-sensitive microgels at fluid interfaces. Though having an isotropic, spherical shape in bulk solution, the microgels become anisotropic upon adsorption. The structure of microgels at interfaces is described by a core-corona morphology. Here, we investigate how changing temperature across the microgel volume phase transition temperature, which leads to swelling/deswelling of the microgels in the aqueous phase, affects the phase behavior within the monolayer. We combine compression isotherms, atomic force microscopy imaging, multiwavelength ellipsometry, and computer simulations. At low compression, the interaction between adsorbed microgels is dominated by their highly stretched corona and the phase behavior of the microgel monolayers is the same. The polymer segments within the interface lose their temperature-sensitivity because of the strong adsorption to the interface. At high compression, however, the portions of the microgels that are located in the aqueous side of the interface become relevant and prevail in the microgel interactions. These portions are able to collapse and, consequently, the isostructural phase transition is altered. Thus, the temperature-dependent swelling perpendicular to the interface ("3D") affects the compressibility parallel to the interface ("2D"). Our results highlight the distinctly different behavior of soft, stimuli-sensitive microgels as compared to rigid nanoparticles.
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http://dx.doi.org/10.1021/acs.langmuir.9b02498DOI Listing
December 2019

Creating gradients of amyloid fibrils from the liquid-liquid interface.

Soft Matter 2019 Oct;15(42):8437-8440

Department of Health Sciences and Technology, Swiss Federal Institut of Technology Zürich, Schmelzbergstrasse 9, 8092 Zürich, Switzerland.

We report a method to deposit amyloid fibrils on a substrate creating gradients in orientation and coverage on demand. For this purpose, we adapt a colloidal self-assembly method at liquid-liquid interfaces to deposit amyloid fibrils on a substrate from the water-hexane interface, while simultaneously compressing it. The amyloid fibril layers orient perpendicularly to the compression, ranging from isotropic to nematic distributions. We furthermore observe reproducible transitions from a monolayer to a bilayer and from a bilayer to multilayers with increasing surface pressures. The creation of each new layer is accompanied by a systematic drop in the structural order of the system, which is however regained upon further compression. This method shows great potential for overcoming the thin-film engineering challenges associated with the manipulation of sticky amyloid fibrils, and allows their ex situ visualisation under compression at the fluid-fluid interface, a situation relevant to understand the propagation of amyloid-related diseases, their functional role in biological systems, and their potential for technological applications.
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http://dx.doi.org/10.1039/c9sm01826cDOI Listing
October 2019

Mechanical phase inversion of Pickering emulsions via metastable wetting of rough colloids.

Soft Matter 2019 Oct;15(39):7888-7900

Laboratory for Soft Materials and Interfaces, Department of Materials, ETH Zürich, Zürich, Switzerland.

The possibility to invert emulsions from oil-in-water to water-in-oil (or vice versa) in a closed system, i.e. without any formulation change, remains an open fundamental challenge with many opportunities for industrial applications. Here, we propose a mechanism that exploits particle surface roughness to induce metastable wetting and obtain mechanically-responsive Pickering emulsions. We postulate that the phase inversion is driven by an in situ switch of the particle wettability from metastable positions at the interface following the input of controlled mechanical energy. Oil-in-water emulsions can be prepared at low energy using mildly hydrophobic rough colloids, which are dispersed in water and weakly pinned at the interface, and switched to water-in-oil emulsions by a second emulsification at higher energy, which triggers the relaxation of the particle contact angle. The same principle is demonstrated for the complementary emulsions using mildly hydrophilic colloids initially dispersed in oil. Our experiments and simulations support that the delicate interplay between particle surface design during synthesis and the energy of the emulsification process can encode a kinetic pathway for the phase inversion. Both organic and inorganic nanoparticles can be used, allowing for the future implementation of our strategy in a broad range of smart industrial formulations.
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http://dx.doi.org/10.1039/c9sm01352kDOI Listing
October 2019

Stress Contributions in Colloidal Suspensions: The Smooth, the Rough, and the Hairy.

Phys Rev Lett 2019 May;122(21):218001

Department of Materials, ETH Zürich, 8032 Zürich, Switzerland.

The collective properties of colloidal suspensions, including their rheology, reflect an interplay between colloidal and hydrodynamic forces. The surface characteristics of the particles play a crucial role, in particular, for applications in which interparticle distances become small, i.e., at high concentrations or in aggregates. In this Letter, we directly investigate this interplay via the linear viscoelastic response of the suspensions in the high-frequency regime, using particles with controlled surface topographies, ranging from smooth to hairy and rough particles. We focus directly on the stresses at the particle level and reveal a strong impact of the surface topography on the short-range interactions, both dissipative and elastic. As the particle topography becomes more complex, the local stresses depend on how the topography is generated. The findings in this Letter, in particular, show how changes in topography can both screen or enhance the dissipation, which can be used to engineer the properties of dense or aggregated suspensions.
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http://dx.doi.org/10.1103/PhysRevLett.122.218001DOI Listing
May 2019

Tuning Interparticle Hydrogen Bonding in Shear-Jamming Suspensions: Kinetic Effects and Consequences for Tribology and Rheology.

J Phys Chem Lett 2019 Apr 27;10(8):1663-1668. Epub 2019 Mar 27.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials , ETH Zurich , 8093 Zurich , Switzerland.

The reversible shear-induced solidification of dense suspensions, known as shear jamming, critically depends on frictional interparticle contacts. Recently, it was shown that shear jamming can be strongly affected by molecular-scale interactions between particles, e.g., by chemically controlling their propensity for hydrogen bonding. However, hydrogen bonding not only enhances interparticle friction but also introduces (reversible) adhesion, whose interplay with friction in shear-jamming systems has so far remained unclear. Here, we present atomic force microscopy studies to assess interparticle adhesion, its relationship to friction, and how these attributes are influenced by urea, a molecule that interferes with hydrogen bonding. We characterize the kinetics of this process with nuclear magnetic resonance, relating it to the time dependence of the macroscopic flow behavior with rheological measurements. We find that time-dependent urea sorption reduces friction and adhesion, causing a reduction in the high-shear viscosity. These results extend our mechanistic understanding of chemical effects on the nature of shear jamming, promising new avenues for fundamental studies and applications alike.
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http://dx.doi.org/10.1021/acs.jpclett.9b00135DOI Listing
April 2019

Microgels Adsorbed at Liquid-Liquid Interfaces: A Joint Numerical and Experimental Study.

ACS Nano 2019 Apr 19;13(4):4548-4559. Epub 2019 Mar 19.

CNR Institute for Complex Systems, Uos Sapienza , Piazzale Aldo Moro 2 , 00185 Roma , Italy.

Soft particles display highly versatile properties with respect to hard colloids and even more so at fluid-fluid interfaces. In particular, microgels, consisting of a cross-linked polymer network, are able to deform and flatten upon adsorption at the interface due to the balance between surface tension and internal elasticity. Despite the existence of experimental results, a detailed theoretical understanding of this phenomenon is still lacking due to the absence of appropriate microscopic models. In this work, we propose an advanced modeling of microgels at a flat water/oil interface. The model builds on a realistic description of the internal polymeric architecture and single-particle properties of the microgel and is able to reproduce its experimentally observed shape at the interface. Complementing molecular dynamics simulations with in situ cryo-electron microscopy experiments and atomic force microscopy imaging after Langmuir-Blodgett deposition, we compare the morphology of the microgels for different values of the cross-linking ratios. Our model allows for a systematic microscopic investigation of soft particles at fluid interfaces, which is essential to develop predictive power for the use of microgels in a broad range of applications, including the stabilization of smart emulsions and the versatile patterning of surfaces.
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http://dx.doi.org/10.1021/acsnano.9b00390DOI Listing
April 2019

Tunable 2D binary colloidal alloys for soft nanotemplating.

Nanoscale 2018 Dec 28;10(47):22189-22195. Epub 2018 Nov 28.

Laboratory for Interfaces, Soft matter and Assembly, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland.

The realization of non-close-packed nanoscale patterns with multiple feature sizes and length scales via colloidal self-assembly is a highly challenging task. We demonstrate here the creation of a variety of tunable particle arrays by harnessing the sequential self-assembly and deposition of two differently sized microgel particles at the fluid-fluid interface. The two-step process is essential to achieve a library of 2D binary colloidal alloys, which are kinetically inaccessible by direct co-assembly. These versatile binary patterns can be exploited for a range of end-uses. Here we show that they can for instance be transferred to silicon substrates, where they act as masks for the metal-assisted chemical etching of binary arrays of vertically aligned silicon nanowires (VA-SiNWs) with fine geometrical control. In particular, continuous binary gradients in both NW spacing and height can be achieved. Notably, these binary VA-SiNW platforms exhibit interesting anti-reflective properties in the visible range, in agreement with simulations. The proposed strategy can also be used for the precise placement of metallic nanoparticles in non-close-packed arrays. Sequential depositions of soft particles enable therefore the exploration of complex binary patterns, e.g. for the future development of substrates for biointerfaces, catalysis and controlled wetting.
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http://dx.doi.org/10.1039/c8nr07059hDOI Listing
December 2018

Dynamics and Wetting Behavior of Core-Shell Soft Particles at a Fluid-Fluid Interface.

Langmuir 2018 12 3;34(50):15370-15382. Epub 2018 Dec 3.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials , ETH Zürich , Vladimir-Prelog-Weg 5 , 8093 Zürich , Switzerland.

We investigate the conformation, position, and dynamics of core-shell nanoparticles (CSNPs) composed of a silica core encapsulated in a cross-linked poly( N-isopropylacrylamide) shell at a water-oil interface for a systematic range of core sizes and shell thicknesses. We first present a free-energy model that we use to predict the CSNP wetting behavior at the interface as a function of its geometrical and compositional properties in the bulk phases, which is in good agreement with our experimental data. Remarkably, based on the knowledge of the polymer shell deformability, the equilibrium particle position relative to the interface plane, an often elusive experimental quantity, can be extracted by measuring its radial dimensions after adsorption. For all the systems studied here, the interfacial dimensions are always larger than in bulk and the particle core resides in a configuration, wherein it just touches the interface or is fully immersed in water. Moreover, the stretched shell induces a larger viscous drag at the interface, which appears to depend solely on the interfacial dimensions, irrespective of the portion of the CSNP surface exposed to the two fluids. Our findings indicate that tailoring the architecture of CSNPs can be used to control their properties at the interface, as of interest for applications including emulsion stabilization and nanopatterning.
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http://dx.doi.org/10.1021/acs.langmuir.8b03048DOI Listing
December 2018

Active Atoms and Interstitials in Two-Dimensional Colloidal Crystals.

Phys Rev Lett 2018 Jun;120(26):268004

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Zurich 8093, Switzerland.

We study experimentally and numerically the motion of a self-phoretic active particle in two-dimensional loosely packed colloidal crystals at fluid interfaces. Two scenarios emerge depending on the interactions between the active particle and the lattice: the active particle either navigates throughout the crystal as an interstitial or is part of the lattice and behaves as an active atom. Active interstitials undergo a run-and-tumble-like motion, with the passive colloids of the crystal acting as tumbling sites. Instead, active atoms exhibit an intermittent motion, stemming from the interplay between the periodic potential landscape of the passive crystal and the particle's self-propulsion. Our results constitute the first step towards the realization of non-close-packed crystalline phases with internal activity.
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http://dx.doi.org/10.1103/PhysRevLett.120.268004DOI Listing
June 2018

Roughness-dependent tribology effects on discontinuous shear thickening.

Proc Natl Acad Sci U S A 2018 05 1;115(20):5117-5122. Epub 2018 May 1.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland;

Surface roughness affects many properties of colloids, from depletion and capillary interactions to their dispersibility and use as emulsion stabilizers. It also impacts particle-particle frictional contacts, which have recently emerged as being responsible for the discontinuous shear thickening (DST) of dense suspensions. Tribological properties of these contacts have been rarely experimentally accessed, especially for nonspherical particles. Here, we systematically tackle the effect of nanoscale surface roughness by producing a library of all-silica, raspberry-like colloids and linking their rheology to their tribology. Rougher surfaces lead to a significant anticipation of DST onset, in terms of both shear rate and solid loading. Strikingly, they also eliminate continuous thickening. DST is here due to the interlocking of asperities, which we have identified as "stick-slip" frictional contacts by measuring the sliding of the same particles via lateral force microscopy (LFM). Direct measurements of particle-particle friction therefore highlight the value of an engineering-tribology approach to tuning the thickening of suspensions.
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http://dx.doi.org/10.1073/pnas.1801066115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5960318PMC
May 2018

Capillary assembly as a tool for the heterogeneous integration of micro- and nanoscale objects.

Soft Matter 2018 Apr;14(16):2978-2995

IBM Research - Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland.

During the past decade, capillary assembly in topographical templates has evolved into an efficient method for the heterogeneous integration of micro- and nano-scale objects on a variety of surfaces. This assembly route has been applied to a large spectrum of materials of micrometer to nanometer dimensions, supplied in the form of aqueous colloidal suspensions. Using systems produced via bulk synthesis affords a huge flexibility in the choice of materials, holding promise for the realization of novel superior devices in the fields of optics, electronics and health, if they can be integrated into surface structures in a fast, simple, and reliable way. In this review, the working principles of capillary assembly and its fundamental process parameters are first presented and discussed. We then examine the latest developments in template design and tool optimization to perform capillary assembly in more robust and efficient ways. This is followed by a focus on the broad range of functional materials that have been realized using capillary assembly, from single components to large-scale heterogeneous multi-component assemblies. We then review current applications of capillary assembly, especially in optics, electronics, and in biomaterials. We conclude with a short summary and an outlook for future developments.
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http://dx.doi.org/10.1039/c7sm02496gDOI Listing
April 2018

Detachment of Rough Colloids from Liquid-Liquid Interfaces.

Langmuir 2018 04 11;34(16):4861-4873. Epub 2018 Apr 11.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials , ETH Zurich , Vladimir-Prelog Weg 5 , 8093 Zürich , Switzerland.

Particle surface roughness and chemistry play a pivotal role in the design of new particle-based materials. Although the adsorption of rough particles has been studied in the literature, desorption of such particles remains poorly understood. In this work, we specifically focus on the detachment of rough and chemically modified raspberry-like microparticles from water/oil interfaces using colloidal-probe atomic force microscopy. We observe different contact-line dynamics occurring upon particle detachment (pinning vs sliding), depending on both the particle roughness and surface modification. In general, surface roughness leads to a reduction of the desorption force of hydrophobic particles into the oil and provides a multitude of pinning points that can be accessed by applying different loads. Our results hence suggest future strategies for stabilization and destabilization of Pickering emulsions and foams.
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http://dx.doi.org/10.1021/acs.langmuir.8b00327DOI Listing
April 2018

Programmable Assembly of Hybrid Nanoclusters.

Langmuir 2018 02 6;34(7):2481-2488. Epub 2018 Feb 6.

Laboratory for Interfaces, Soft Matter, and Assembly, Department of Materials, ETH Zurich , Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.

Hybrid nanoparticle clusters (often metallic) are interesting plasmonic materials with tunable resonances and a near-field electromagnetic enhancement at interparticle junctions. Therefore, in recent years, we have witnessed a surge in both the interest in these materials and the efforts to obtain them. However, a versatile fabrication of hybrid nanoclusters, that is, combining more than one material, still remains an open challenge. Current lithographical or self-assembly methods are limited to the preparation of hybrid clusters with up to two different materials and typically to the fabrication of hybrid dimers. Here, we provide a novel strategy to deposit and align not only hybrid dimers but also hybrid nanoclusters possessing more complex shapes and compositions. Our strategy is based on the downscaling of sequential capillarity-assisted particle assembly over topographical templates. As a proof of concept, we demonstrate dimers, linear trimers, and 2D nanoclusters with programmable compositions from a range of metallic nanoparticles. Our process does not rely on any specific chemistry and can be extended to a large variety of particles and shapes. The template also simultaneously aligns the hybrid (often anisotropic) nanoclusters, which could facilitate device integration, for example, for optical readout after transfer to other substrates by a printing step. We envisage that this new fabrication route will enable the assembly and positioning of complex hybrid nanoclusters of different functional nanoparticles to study coupling effects not only locally but also at larger scales for new nanoscale optical devices.
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http://dx.doi.org/10.1021/acs.langmuir.7b03944DOI Listing
February 2018

Direct observation of impact propagation and absorption in dense colloidal monolayers.

Proc Natl Acad Sci U S A 2017 11 30;114(46):12150-12155. Epub 2017 Oct 30.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland;

Dense colloidal suspensions can propagate and absorb large mechanical stresses, including impacts and shocks. The wave transport stems from the delicate interplay between the spatial arrangement of the structural units and solvent-mediated effects. For dynamic microscopic systems, elastic deformations of the colloids are usually disregarded due to the damping imposed by the surrounding fluid. Here, we study the propagation of localized mechanical pulses in aqueous monolayers of micron-sized particles of controlled microstructure. We generate extreme localized deformation rates by exciting a target particle via pulsed-laser ablation. In crystalline monolayers, stress propagation fronts take place, where fast-moving particles ( approximately a few meters per second) are aligned along the symmetry axes of the lattice. Conversely, more viscous solvents and disordered structures lead to faster and isotropic energy absorption. Our results demonstrate the accessibility of a regime where elastic collisions also become relevant for suspensions of microscopic particles, behaving as "billiard balls" in a liquid, in analogy with regular packings of macroscopic spheres. We furthermore quantify the scattering of an impact as a function of the local structural disorder.
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http://dx.doi.org/10.1073/pnas.1712266114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5699069PMC
November 2017

Enhanced Second-Harmonic Generation from Sequential Capillarity-Assisted Particle Assembly of Hybrid Nanodimers.

Nano Lett 2017 09 9;17(9):5381-5388. Epub 2017 Aug 9.

Optical Nanomaterial Group, Institute for Quantum Electronics, Department of Physics, ETH Zürich , Auguste-Piccard- Hof 1, 8093 Zürich, Switzerland.

We show enhanced second-harmonic generation (SHG) from a hybrid metal-dielectric nanodimer consisting of an inorganic perovskite nanoparticle of barium titanate (BaTiO) coupled to a metallic gold (Au) nanoparticle. BaTiO-Au nanodimers of 100 nm/80 nm sizes are fabricated by sequential capillarity-assisted particle assembly. The BaTiO nanoparticle has a noncentrosymmetric crystalline structure and generates bulk SHG. We use the localized surface plasmon resonance of the gold nanoparticle to enhance the SHG from the BaTiO nanoparticle. We experimentally measure the nonlinear signal from assembled nanodimers and demonstrate an up to 15-fold enhancement compared to a single BaTiO nanoparticle. We further perform numerical simulations of the linear and SHG spectra of the BaTiO-Au nanodimer and show that the gold nanoparticle acts as a nanoantenna at the SHG wavelength.
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http://dx.doi.org/10.1021/acs.nanolett.7b01940DOI Listing
September 2017

Stable in Bulk and Aggregating at the Interface: Comparing Core-Shell Nanoparticles in Suspension and at Fluid Interfaces.

Langmuir 2018 01 16;34(3):886-895. Epub 2017 Aug 16.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zürich , Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland.

Colloidal particles are extensively used to assemble materials from bulk suspensions or after adsorption and confinement at fluid interfaces (e.g., oil-water interfaces). Interestingly, and often underestimated, optimizing interactions for bulk assembly may not lead to the same behavior at fluid interfaces. In this work, we compare model composite nanoparticles with a silica core coated with a poly-N-isopropylacrylamide hydrogel shell in bulk aqueous suspensions and after adsorption at an oil-water interface. Bulk properties are analyzed by confocal differential dynamic microscopy, a recently developed technique that allows one to simultaneously obtain structural and dynamical information up to high volume fractions. The results demonstrate excellent colloidal stability and the absence of aggregation in all cases. The behavior at the interface, investigated by a range of complementary approaches, is instead different. The same hydrogel shells that stabilize the particles in the bulk deform at the interface and induce attractive capillary interactions that lead to aggregation even at very low area fractions (surface coverage). Upon further compression of a particle-laden interface, a structural transition is observed where closely packed particle aggregates form. These findings emphasize the manifestation of different, and possibly unexpected, responses for sterically stabilized nanoparticles in the bulk and upon interfacial confinement.
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http://dx.doi.org/10.1021/acs.langmuir.7b02015DOI Listing
January 2018

Hybrid Colloids Produced by Sequential Capillarity-assisted Particle Assembly: A New Path for Complex Microparticles.

Chimia (Aarau) 2017 Jun;71(6):349-353

Laboratory for Interfaces, Soft Matter, and Assembly Department of Materials, ETH Zurich Vladimir-Prelog-Weg 5, CH-8093 Zurich;, Email:

Colloidal particles have long been under the spotlight of a very diverse research community, given their ubiquitous presence in a broad class of materials and processes, and their pivotal role as model systems. More recently, intense efforts have been devoted to the development of micro- and nanoparticles combining multiple materials in objects with a controlled architecture, hence introducing multiple functionalities and a prescribed symmetry for interactions. These particles are often called hybrid colloids or colloidal molecules, given the analogy with classical molecules presenting well defined structures and chemical compositions. Here, we review the recent progress made in our group to fabricate a broad library of hybrid colloids exploiting a novel assembly route, which uses capillary forces at the moving edge of an evaporating droplet for the sequential composition of colloidal clusters, whose geometry and chemistry can be independently programmed.
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http://dx.doi.org/10.2533/chimia.2017.349DOI Listing
June 2017

Universal emulsion stabilization from the arrested adsorption of rough particles at liquid-liquid interfaces.

Nat Commun 2017 06 7;8:15701. Epub 2017 Jun 7.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog Weg 5, 8093 Zürich, Switzerland.

Surface heterogeneities, including roughness, significantly affect the adsorption, motion and interactions of particles at fluid interfaces. However, a systematic experimental study, linking surface roughness to particle wettability at a microscopic level, is currently missing. Here we synthesize a library of all-silica microparticles with uniform surface chemistry, but tuneable surface roughness and study their spontaneous adsorption at oil-water interfaces. We demonstrate that surface roughness strongly pins the particles' contact lines and arrests their adsorption in long-lived metastable positions, and we directly measure the roughness-induced interface deformations around isolated particles. Pinning imparts tremendous contact angle hysteresis, which can practically invert the particle wettability for sufficient roughness, irrespective of their chemical nature. As a unique consequence, the same rough particles stabilize both water-in-oil and oil-in-water emulsions depending on the phase they are initially dispersed in. These results both shed light on fundamental phenomena concerning particle adsorption at fluid interfaces and indicate future design rules for particle-based emulsifiers.
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http://dx.doi.org/10.1038/ncomms15701DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5467241PMC
June 2017

Hybrid colloidal microswimmers through sequential capillary assembly.

Soft Matter 2017 Jun;13(23):4252-4259

Laboratory for Interfaces, Soft Matter, and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.

Active colloids, also known as artificial microswimmers, are self-propelled micro- and nanoparticles that convert uniform sources of fuel (e.g. chemical) or uniform external driving fields (e.g. magnetic or electric) into directed motion by virtue of asymmetry in their shape or composition. These materials are currently attracting enormous scientific attention as models for out-of-equilibrium systems and with the promise to be used as micro- and nanoscale devices. However, current fabrication of active colloids is limited in the choice of available materials, geometries, and modes of motion. Here, we use sequential capillarity-assisted particle assembly (sCAPA) to link microspheres of different materials into hybrid clusters of prescribed shapes ("colloidal molecules") that can actively translate, circulate and rotate powered by asymmetric electro-hydrodynamic flows. We characterize the active motion of the clusters and highlight the range of parameters (composition and shape) that can be used to tune their trajectories. Further engineering provides active colloids that switch motion under external triggers or perform simple pick-up and transport tasks. By linking their design, realization and characterization, our findings enable and inspire both physicists and engineers to create customized active colloids to explore novel fundamental phenomena in active matter and to investigate materials and propulsion schemes that are compatible with future applications.
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http://dx.doi.org/10.1039/c7sm00443eDOI Listing
June 2017

Adsorption of Ellipsoidal Particles at Liquid-Liquid Interfaces.

Langmuir 2017 03 13;33(11):2689-2697. Epub 2017 Mar 13.

Department of Chemical Engineering, KU Leuven , B-3001 Leuven, Belgium.

The adsorption of particles at liquid-liquid interfaces is of great scientific and technological importance. In particular, for nonspherical particles, the capillary forces that drive adsorption vary with position and orientation, and complex adsorption pathways have been predicted by simulations. On the basis of the latter, it has been suggested that the timescales of adsorption are determined by a balance between capillary and viscous forces. However, several recent experimental results point out the role of contact line pinning in the adsorption of particles to interfaces and even suggest that the adsorption dynamics and pathways are completely determined by the latter, with the timescales of adsorption being determined solely by particle characteristics. In the present work, the adsorption trajectories of model ellipsoidal particles are investigated experimentally using cryo-SEM and by monitoring the altitudinal orientation angle using high-speed confocal microscopy. By varying the viscosity and the viscosity jump across the interfaces, we specifically interrogate the role of viscous forces.
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http://dx.doi.org/10.1021/acs.langmuir.6b03534DOI Listing
March 2017

Colloidal polycrystalline monolayers under oscillatory shear.

Phys Rev E 2017 Jan 30;95(1-1):012610. Epub 2017 Jan 30.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, CH-8093, Zurich, Switzerland.

In this paper we probe the structural response to oscillatory shear deformations of polycrystalline monolayers of soft repulsive colloids with varying area fraction over a broad range of frequencies and amplitudes. The particles are confined at a fluid interface, sheared using a magnetic microdisk, and imaged through optical microscopy. The structural and mechanical response of soft materials is highly dependent on their microstructure. If crystals are well understood and deform through the creation and mobilization of specific defects, the situation is much more complex for disordered jammed materials, where identifying structural motifs defining plastically rearranging regions remains an elusive task. Our materials fall between these two classes and allow the identification of clear pathways for structural evolution. In particular, we demonstrate that large enough strains are able to fluidize the system, identifying critical strains that fulfill a local Lindemann criterion. Conversely, smaller strains lead to localized and erratic irreversible particle rearrangements due to the motion of structural defects. In this regime, oscillatory shear promotes defect annealing and leads to the growth of large crystalline domains. Numerical simulations help identify the population of rearranging particles with those exhibiting the largest deviatoric stresses and indicate that structural evolution proceeds towards the minimization of the stress stored in the system. The particles showing high deviatoric stresses are localized around grain boundaries and defects, providing a simple criterion to spot regions likely to rearrange plastically under oscillatory shear.
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http://dx.doi.org/10.1103/PhysRevE.95.012610DOI Listing
January 2017