Publications by authors named "Paul van der Schoot"

110 Publications

Transient response and domain formation in electrically deforming liquid crystal networks.

Soft Matter 2022 May 11;18(18):3594-3604. Epub 2022 May 11.

Eindhoven University of Technology, Department of Applied Physics, 5612AZ Eindhoven, The Netherlands.

Recently, three distinct, well-separated transient regimes were discovered in the dynamics of the volume expansion of shape-shifting liquid crystal network films in response to the switching on of an alternating electric field [Van der Kooij , , 2019, , 1]. Employing a spatially resolved, time-dependent Landau theory that couples local volume generation to the degree of orientational order of mesogens that are part of a viscoelastic network, we are able to offer a physical explanation for the existence of three time scales. We find that the initial response is dominated by overcoming the impact of thermal noise, after which the top of the film expands, followed by a permeation of this response into the bulk region. An important signature of our predictions is a significant dependence of the three time scales on the film thickness, where we observe a clear thin-film-to-bulk transition. The point of transition coincides with the emergence of spatial inhomogeneities in the bulk of the film in the form of domains separated by regions of suppressed expansion. This ultimately gives rise to variations in the steady-state overall expansion of the film and may lead to uncontrolled patterning. According to our model, domain formation can be suppressed by (1) decreasing the thickness of the as-prepared film, (2) increasing the linear dimensions of the mesogens, or (3) their degree of orientational order when cross-linked into the network. Our findings provide a handle to achieve finer control over the actuation of smart liquid crystal network coatings.
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http://dx.doi.org/10.1039/d2sm00125jDOI Listing
May 2022

Structure of nematic tactoids of hard rods.

J Chem Phys 2022 Mar;156(10):104501

Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.

We study by means of Monte Carlo simulations the internal structure of nematic droplets or tactoids formed by hard, rod-like particles in a gas of spherical ghost particles that act as depletion agents for the rods. We find that the shape and internal structure of tactoids are strongly affected by the size of the droplets. The monotonically increasing degree of nematic order with increasing particle density that characterizes the bulk nematic phase is locally violated and more so the smaller the tactoid. We also investigate the impact of an external quadrupolar alignment field on tactoids and find that this tends to make the director field more uniform, but not to very significantly increase the tactoid's aspect ratio. This agrees with recent theoretical predictions yet is at variance with experimental observations and dynamical simulations. We explain this discrepancy in terms of competing relaxation times.
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http://dx.doi.org/10.1063/5.0078056DOI Listing
March 2022

Connectedness percolation of fractal liquids.

Phys Rev E 2021 Nov;104(5-1):054605

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

We apply connectedness percolation theory to fractal liquids of hard particles, and make use of a Percus-Yevick liquid state theory combined with a geometric connectivity criterion. We find that in fractal dimensions the percolation threshold interpolates continuously between integer-dimensional values, and that it decreases monotonically with increasing (fractal) dimension. The influence of hard-core interactions is significant only for dimensions below three. Finally, our theory incorrectly suggests that a percolation threshold is absent below about two dimensions, which we attribute to the breakdown of the connectedness Percus-Yevick closure.
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http://dx.doi.org/10.1103/PhysRevE.104.054605DOI Listing
November 2021

Controlling permeation in electrically deforming liquid crystal network films: A dynamical Landau theory.

Phys Rev E 2021 Nov;104(5-1):054701

Department of Applied Physics, Eindhoven University of Technology, The Netherlands.

Liquid crystal networks exploit the coupling between the responsivity of liquid crystalline mesogens, e.g., to electric fields, and the (visco)elastic properties of a polymer network. Because of this, these materials have been put forward for a wide array of applications, including responsive surfaces such as artificial skins and membranes. For such applications, the desired functional response must generally be realized under strict geometrical constraints, such as provided by supported thin films. To model such settings, we present a dynamical, spatially heterogeneous Landau-type theory for electrically actuated liquid crystal network films. We find that the response of the liquid crystal network permeates the film from top to bottom, and illustrate how this affects the timescale associated with macroscopic deformation. Finally, by linking our model parameters to experimental quantities, we suggest that the permeation rate can be controlled by varying the aspect ratio of the mesogens and their degree of orientational order when crosslinked into the polymer network, for which we predict a single optimum. Our results contribute specifically to the rational design of future applications involving transport or on-demand release of molecular cargo in liquid crystal network films.
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http://dx.doi.org/10.1103/PhysRevE.104.054701DOI Listing
November 2021

Continuum percolation in colloidal dispersions of hard nanorods in external axial and planar fields.

Soft Matter 2021 Dec 1;17(46):10458-10468. Epub 2021 Dec 1.

Group of Soft Matter and Biological Physics, Eindhoven University of Technology, Eindhoven, The Netherlands.

We present a theoretical study on continuum percolation of rod-like colloidal particles in the presence of axial and planar quadrupole fields. Our work is based on a self-consistent numerical treatment of the connectedness Ornstein-Zernike equation, in conjunction with the Onsager equation that describes the orientational distribution function of particles interacting a hard-core repulsive potential. Our results show that axial and planar quadrupole fields both in principle increase the percolation threshold. By how much depends on a combination of the field strength, the concentration, the aspect ratio of the particles, and percolation criterion. We find that the percolated state can form and break down multiple times with increasing concentration, , it exhibits re-entrance behaviour. Finally, we show that planar fields may induce a high degree of triaxiality in the shape of particle clusters that in actual materials may give rise to highly anisotropic conductivity properties.
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http://dx.doi.org/10.1039/d1sm01408kDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8634899PMC
December 2021

Effect of electric fields on the director field and shape of nematic tactoids.

Phys Rev E 2021 Jun;103(6-1):062703

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Tactoids are spindle-shaped droplets of a uniaxial nematic phase suspended in the coexisting isotropic phase. They are found in dispersions of a wide variety of elongated colloidal particles, including actin, fd virus, carbon nanotubes, vanadium peroxide, and chitin nanocrystals. Recent experiments on tactoids of chitin nanocrystals in water show that electric fields can very strongly elongate tactoids even though the dielectric properties of the coexisting isotropic and nematic phases differ only subtly. We develop a model for partially bipolar tactoids, where the degree of bipolarness of the director field is free to adjust to optimize the sum of the elastic, surface, and Coulomb energies of the system. By means of a combination of a scaling analysis and a numerical study, we investigate the elongation and director field's behavior of the tactoids as a function of their size, the strength of the electric field, the surface tension, anchoring strength, the elastic constants, and the electric susceptibility anisotropy. We find that tactoids cannot elongate significantly due to an external electric field, unless the director field is bipolar or quasibipolar and somehow frozen in the field-free configuration. Presuming that this is the case, we find reasonable agreement with experimental data.
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http://dx.doi.org/10.1103/PhysRevE.103.062703DOI Listing
June 2021

Nearest-neighbor connectedness theory: A general approach to continuum percolation.

Phys Rev E 2021 Apr;103(4-1):042115

Institute of Physics, University of Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany.

We introduce a method to estimate continuum percolation thresholds and illustrate its usefulness by investigating geometric percolation of noninteracting line segments and disks in two spatial dimensions. These examples serve as models for electrical percolation of elongated and flat nanofillers in thin film composites. While the standard contact volume argument and extensions thereof in connectedness percolation theory yield accurate predictions for slender nanofillers in three dimensions, they fail to do so in two dimensions, making our test a stringent one. In fact, neither a systematic order-by-order correction to the standard argument nor invoking the connectedness version of the Percus-Yevick approximation yield significant improvements for either type of particle. Making use of simple geometric considerations, our new method predicts a percolation threshold of ρ_{c}l^{2}≈5.83 for segments of length l, which is close to the ρ_{c}l^{2}≈5.64 found in Monte Carlo simulations. For disks of area a we find ρ_{c}a≈1.00, close to the Monte Carlo result of ρ_{c}a≈1.13. We discuss the shortcomings of the conventional approaches and explain how usage of the nearest-neighbor distribution in our method bypasses those complications.
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http://dx.doi.org/10.1103/PhysRevE.103.042115DOI Listing
April 2021

Impact of the prequench state of binary fluid mixtures on surface-directed spinodal decomposition.

Phys Rev E 2021 Apr;103(4-1):042801

Fluids and Flows Group, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Using lattice Boltzmann simulations we investigate the impact of the amplitude of concentration fluctuations in binary fluid mixtures prior to demixing when in contact with a surface that is preferentially wet by one of the components. We find a bicontinuous structure near the surface for an initial, prequench state of the mixture close to the critical point where the amplitude of concentration fluctuations is large. In contrast, if the initial state of the mixture is not near the critical point and concentration fluctuations are relatively weak, then the morphology is not bicontinuous but remains layered until the very late stages of coarsening. In both cases, it is the morphology of a depletion layer rich in the nonpreferred component that dictates the growth exponent of the thickness of the fluid layer that is in direct contact with the substrate. In the early stages of demixing, we find a growth exponent consistent with a value of 1/4 for a prequench state away from the critical point, which is different from the usual diffusive scaling exponent of 1/3 that we recover for a prequench state close to the critical point. We attribute this to the structure of a depletion layer that is penetrated by tubes of the preferred fluid, connecting the wetting layer to the bulk fluid even in the early stages if the initial state is characterized by concentration fluctuations that are large in amplitude. Furthermore, we find that in the late stages of demixing the flow through these tubes results in significant in-plane concentration variations near the substrate, leading to dropletlike structures with a concentration lower than the average concentration in the wetting layer. This causes a deceleration in the growth of the wetting layer in the very late stages of the demixing. Irrespective of the prequench state of the mixture, the late stages of the demixing process produce the same growth law for the layer thickness, with a scaling exponent of unity usually associated with the impact of hydrodynamic flow fields.
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http://dx.doi.org/10.1103/PhysRevE.103.042801DOI Listing
April 2021

Enhanced ordering in length-polydisperse carbon nanotube solutions at high concentrations as revealed by small angle X-ray scattering.

Soft Matter 2021 May 18;17(20):5122-5130. Epub 2021 Mar 18.

Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA.

Carbon nanotubes (CNTs) are stiff, all-carbon macromolecules with diameters as small as one nanometer and few microns long. Solutions of CNTs in chlorosulfonic acid (CSA) follow the phase behavior of rigid rod polymers interacting via a repulsive potential and display a liquid crystalline phase at sufficiently high concentration. Here, we show that small-angle X-ray scattering and polarized light microscopy data can be combined to characterize quantitatively the morphology of liquid crystalline phases formed in CNT solutions at concentrations from 3 to 6.5% by volume. We find that upon increasing their concentration, CNTs self-assemble into a liquid crystalline phase with a pleated texture and with a large inter-particle spacing that could be indicative of a transition to higher-order liquid crystalline phases. We explain how thermal undulations of CNTs can enhance their electrostatic repulsion and increase their effective diameter by an order of magnitude. By calculating the critical concentration, where the mean amplitude of undulation of an unconstrained rod becomes comparable to the rod spacing, we find that thermal undulations start to affect steric forces at concentrations as low as the isotropic cloud point in CNT solutions.
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http://dx.doi.org/10.1039/d0sm02253eDOI Listing
May 2021

Virus Mechanics under Molecular Crowding.

J Phys Chem B 2021 02 12;125(7):1790-1798. Epub 2021 Feb 12.

Department of Chemistry, Indiana University, Bloomington, Bloomington, Indiana 47405, United States.

Viruses avoid exposure of the viral genome to harmful agents with the help of a protective protein shell known as the capsid. A secondary effect of this protective barrier is that macromolecules that may be in high concentration on the outside cannot freely diffuse across it. Therefore, inside the cell and possibly even outside, the intact virus is generally under a state of osmotic stress. Viruses deal with this type of stress in various ways. In some cases, they might harness it for infection. However, the magnitude and influence of osmotic stress on virus physical properties remains virtually unexplored for single-stranded RNA viruses-the most abundant class of viruses. Here, we report on how a model system for the positive-sense RNA icosahedral viruses, brome mosaic virus (BMV), responds to osmotic pressure. Specifically, we study the mechanical properties and structural stability of BMV under controlled molecular crowding conditions. We show that BMV is mechanically reinforced under a small external osmotic pressure but starts to yield after a threshold pressure is reached. We explain this mechanochemical behavior as an effect of the molecular crowding on the entropy of the "breathing" fluctuation modes of the virus shell. The experimental results are consistent with the viral RNA imposing a small negative internal osmotic pressure that prestresses the capsid. Our findings add a new line of inquiry to be considered when addressing the mechanisms of viral disassembly inside the crowded environment of the cell.
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http://dx.doi.org/10.1021/acs.jpcb.0c10947DOI Listing
February 2021

Nonmonotonic swelling and compression dynamics of hydrogels in polymer solutions.

Phys Rev E 2020 Dec;102(6-1):062606

Department of Mechanical Engineering, Materials Technology, Eindhoven University of Technology, 5600MB Eindhoven, Netherlands.

Hydrogels are sponge-like materials that can absorb or expel significant amounts of water. Swelling up from a dried state, they can swell up more than a hundredfold in volume, with the kinetics and the degree of swelling depending sensitively on the physicochemical properties of both the polymer network and the aqueous solvent. In particular, the presence of dissolved macromolecules in the background liquid can have a significant effect, as the macromolecules can exert an additional external osmotic pressure on the hydrogel material, thereby reducing the degree of swelling. In this paper, we have submerged dry hydrogel particles in polymer solutions containing large and small macromolecules. Interestingly, for swelling in the presence of large macromolecules we observe a concentration-dependent overshoot behavior, where the particle volume first continuously increases toward a maximum, after which it decreases again, reaching a lower, equilibrium value. In the presence of smaller macromolecules we do not observe this intriguing overshoot behavior, but instead observe a rapid growth followed by a slowed-down growth. To account for the observed overshoot behavior, we realize that the macromolecules entering the hydrogel network not only lead to a reduction of the osmotic pressure difference, but their presence within the network also affects the swelling behavior through a modification of the solvent-polymer interactions. In this physical picture of the swelling process, the net amount of volume change should thus depend on the magnitudes of both the reduction in osmotic pressure and the change in effective solvent quality associated with the macromolecules entering the pores of the hydrogel network. We develop a phenomenological model that incorporates both of these effects. Using this model we are able to account for both the swelling and compression kinetics of hydrogels within aqueous polymer solutions, as a function of the size of the dissolved macromolecules and of their effect on the effective solvent quality.
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http://dx.doi.org/10.1103/PhysRevE.102.062606DOI Listing
December 2020

Dynamical Landau-de Gennes theory for electrically-responsive liquid crystal networks.

Phys Rev E 2020 Oct;102(4-1):042703

Department of Applied Physics, Eindhoven University of Technology, The Netherlands.

Liquid crystal networks combine the orientational order of liquid crystals with the elastic properties of polymer networks, leading to a vast application potential in the field of responsive coatings, e.g., for haptic feedback, self-cleaning surfaces, and static and dynamic pattern formation. Recent experimental work has further paved the way toward such applications by realizing the fast and reversible surface modulation of a liquid crystal network coating upon in-plane actuation with an AC electric field [Liu, Tito, and Broer, Nat. Commun. 8, 1526 (2017)10.1038/s41467-017-01448-w]. Here, we construct a Landau-type theory for electrically-responsive liquid crystal networks and perform molecular dynamics simulations to explain the findings of these experiments and inform on rational design strategies. Qualitatively, the theory agrees with our simulations and reproduces the salient experimental features. We also provide a set of testable predictions: the aspect ratio of the nematogens, their initial orientational order when cross-linked into the polymer network, and the cross-linking fraction of the network all increase the plasticization time required for the film to macroscopically deform. We demonstrate that the dynamic response to oscillating electric fields is characterized by two resonances, which can likewise be influenced by varying these parameters, providing an experimental handle to fine-tune device design.
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http://dx.doi.org/10.1103/PhysRevE.102.042703DOI Listing
October 2020

Combined Force-Torque Spectroscopy of Proteins by Means of Multiscale Molecular Simulation.

Biophys J 2020 12 27;119(11):2240-2250. Epub 2020 Oct 27.

Theory of Polymers and Soft Matter, Eindhoven University of Technology, Eindhoven, the Netherlands; Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, the Netherlands.

Assessing the structural properties of large proteins is important to gain an understanding of their function in, e.g., biological systems or biomedical applications. We propose a method to examine the mechanical properties of proteins subject to applied forces by means of multiscale simulation. Both stretching and torsional forces are considered, and these may be applied independently of each other. As a proof of principle, we apply torsional forces to a coarse-grained continuum model of the antibody protein immunoglobulin G using fluctuating finite element analysis and use it to identify the area of strongest deformation. This region is essential to the torsional properties of the molecule as a whole because it represents the softest, most deformable domain. Zooming in, this part of the molecule is subjected to torques and stretching forces using molecular dynamics simulations on an atomistically resolved level to investigate its torsional properties. We calculate the torsional resistance as a function of the rotation of the domain while subjecting it to various stretching forces. From this, we assess how the measured twist-torque profiles develop with increasing stretching force and show that they exhibit torsion stiffening, in qualitative agreement with experimental findings. We argue that combining the twist-torque profiles for various stretching forces effectively results in a combined force-torque spectroscopy analysis, which may serve as a mechanical signature for a biological macromolecule.
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http://dx.doi.org/10.1016/j.bpj.2020.09.039DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7732774PMC
December 2020

Geometric percolation of hard nanorods: The interplay of spontaneous and externally induced uniaxial particle alignment.

J Chem Phys 2020 Feb;152(6):064902

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 3500 MB Eindhoven, The Netherlands.

We present a numerical study on geometric percolation in liquid dispersions of hard slender colloidal particles subject to an external orienting field. In the formulation and liquid-state processing of nanocomposite materials, particle alignment by external fields such as electric, magnetic, or flow fields is practically inevitable and often works against the emergence of large nanoparticle networks. Using continuum percolation theory in conjunction with Onsager theory, we investigate how the interplay between externally induced alignment and the spontaneous symmetry breaking of the uniaxial nematic phase affects cluster formation in nanoparticle dispersions. It is known that particle alignment by means of a density increase or by an external field may result in a breakdown of an already percolating network. As a result, percolation can be limited to a small region of the phase diagram only. Here, we demonstrate that the existence and shape of such a "percolation island" in the phase diagram crucially depends on the connectivity length-a critical distance defining direct connections between neighboring particles. For some values of the connectivity range, we observe unusual re-entrance effects, in which a system-spanning network forms and breaks down multiple times with increasing particle density.
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http://dx.doi.org/10.1063/1.5141481DOI Listing
February 2020

Unusual geometric percolation of hard nanorods in the uniaxial nematic liquid crystalline phase.

Phys Rev E 2019 Dec;100(6-1):062129

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 3500 MB Eindhoven, The Netherlands.

We investigate by means of continuum percolation theory and Monte Carlo simulations how spontaneous uniaxial symmetry breaking affects geometric percolation in dispersions of hard rodlike particles. If the particle aspect ratio exceeds about 20, percolation in the nematic phase can be lost upon adding particles to the dispersion. This contrasts with percolation in the isotropic phase, where a minimum particle loading is always required to obtain system-spanning clusters. For sufficiently short rods, percolation in the uniaxial nematic mimics that of the isotropic phase, where the addition of particles always aids percolation. For aspect ratios between 20 and infinity, but not including infinity, we find reentrance behavior: percolation in the low-density nematic may be lost upon increasing the amount of nanofillers but can be regained by the addition of even more particles to the suspension. Our simulation results for aspect ratios of 5, 10, 20, 50, and 100 strongly support our theoretical predictions, with almost quantitative agreement. We show that a different closure of the connectedness Ornstein-Zernike equation, inspired by scaled particle theory, is as least as accurate in predicting the percolation threshold as the Parsons-Lee closure, which effectively describes the impact of many-body direct contacts.
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http://dx.doi.org/10.1103/PhysRevE.100.062129DOI Listing
December 2019

Self-organization of tip-functionalized elongated colloidal particles.

Phys Rev E 2019 Oct;100(4-1):042702

Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands and Institute for Theoretical Physics, Utrecht University, 3584 CC Utrecht, The Netherlands.

Weakly attractive interactions between the tips of rodlike colloidal particles affect their liquid-crystal phase behavior due to a subtle interplay between enthalpy and entropy. Here we employ molecular dynamics simulations on semiflexible, repulsive bead-spring chains where one of the two end beads attract each other. We calculate the phase diagram as a function of both the volume fraction of the chains and the strength of the attractive potential. We identify a large number of phases that include isotropic, nematic, smectic-A, smectic-B, and crystalline states. For tip attraction energies lower than the thermal energy, our results are qualitatively consistent with experimental findings: We find that an increase of the attraction strength shifts the nematic to smectic-A phase transition to lower volume fractions, with only minor effect on the stability of the other phases. For sufficiently strong tip attraction, the nematic phase disappears completely, in addition leading to the destabilization of the isotropic phase. In order to better understand the underlying physics of these phenomena, we also investigate the clustering of the particles at their attractive tips and the effective molecular field experienced by the particles in the smectic-A phase. Based on these results, we argue that the clustering of the tips only affects the phase stability if lamellar structures ("micelles") are formed. We find that an increase of the attraction strength increases the degree of order in the layered phases. Interestingly, we also find evidence for the existence of an antiferroelectric smectic-A phase transition induced by the interaction between the tips. A simple Maier-Saupe-McMillan model confirms our findings.
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http://dx.doi.org/10.1103/PhysRevE.100.042702DOI Listing
October 2019

Real-Time Assembly of Viruslike Nucleocapsids Elucidated at the Single-Particle Level.

Nano Lett 2019 08 1;19(8):5746-5753. Epub 2019 Aug 1.

Moleculaire Biofysica, Zernike Instituut , Rijksuniversiteit Groningen , 9712 CP Groningen , The Netherlands.

While the structure of a multitude of viral particles has been resolved to atomistic detail, their assembly pathways remain largely elusive. Key unresolved issues are particle nucleation, particle growth, and the mode of genome compaction. These issues are difficult to address in bulk approaches and are effectively only accessible by the real-time tracking of assembly dynamics of individual particles. This we do here by studying the assembly into rod-shaped viruslike particles (VLPs) of artificial capsid polypeptides. Using fluorescence optical tweezers, we establish that small oligomers perform one-dimensional diffusion along the DNA. Larger oligomers are immobile and nucleate VLP growth. A multiplexed acoustic force spectroscopy approach reveals that DNA is compacted in regular steps, suggesting packaging via helical wrapping into a nucleocapsid. By reporting how real-time assembly tracking elucidates viral nucleation and growth principles, our work opens the door to a fundamental understanding of the complex assembly pathways of both VLPs and naturally evolved viruses.
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http://dx.doi.org/10.1021/acs.nanolett.9b02376DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6696885PMC
August 2019

Supramolecular copolymers predominated by alternating order: Theory and application.

J Chem Phys 2019 Jul;151(1):014902

Institute of Physics, Johannes Gutenberg-University, Staudingerweg 7-9, 55128 Mainz, Germany.

We investigate the copolymerization behavior of a two-component system into quasilinear self-assemblies under conditions that interspecies binding is favored over identical species binding. The theoretical framework is based on a coarse-grained self-assembled Ising model with nearest neighbor interactions. In Ising language, such conditions correspond to the antiferromagnetic case giving rise to copolymers with predominantly alternating configurations. In the strong coupling limit, we show that the maximum fraction of polymerized material and the average length of strictly alternating copolymers depend on the stoichiometric ratio and the activation free energy of the more abundant species. They are substantially reduced when the stoichiometric ratio noticeably differs from unity. Moreover, for stoichiometric ratios close to unity, the copolymerization critical concentration is remarkably lower than the homopolymerization critical concentration of either species. We further analyze the polymerization behavior for a finite and negative coupling constant and characterize the composition of supramolecular copolymers. Our theoretical insights rationalize experimental results of supramolecular polymerization of oppositely charged monomeric species in aqueous solutions.
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http://dx.doi.org/10.1063/1.5097577DOI Listing
July 2019

Directing Liquid Crystalline Self-Organization of Rodlike Particles through Tunable Attractive Single Tips.

Phys Rev Lett 2019 Mar;122(12):128008

Centre de Recherche Paul-Pascal, CNRS and Université de Bordeaux, 115 Avenue Schweitzer, F-33600 Pessac, France.

Dispersions of rodlike colloidal particles exhibit a plethora of liquid crystalline states, including nematic, smectic A, smectic B, and columnar phases. This phase behavior can be explained by presuming the predominance of hard-core volume exclusion between the particles. We show here how the self-organization of rodlike colloids can be controlled by introducing a weak and highly localized directional attractive interaction between one of the ends of the particles. This has been performed by functionalizing the tips of filamentous viruses by means of regioselectively grafting fluorescent dyes onto them, resulting in a hydrophobic patch whose attraction can be tuned by varying the number of bound dye molecules. We show, in agreement with our computer simulations, that increasing the single tip attraction stabilizes the smectic phase at the expense of the nematic phase, leaving all other liquid crystalline phases invariant. For a sufficiently strong tip attraction, the nematic state may be suppressed completely to get a direct isotropic liquid-to-smectic phase transition. Our findings provide insights into the rational design of building blocks for functional structures formed at low densities.
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http://dx.doi.org/10.1103/PhysRevLett.122.128008DOI Listing
March 2019

Connectivity, Not Density, Dictates Percolation in Nematic Liquid Crystals of Slender Nanoparticles.

Phys Rev Lett 2019 Mar;122(9):097801

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 3500 MB Eindhoven, Netherlands.

We show by means of continuum theory and simulations that geometric percolation in uniaxial nematics of hard slender particles is fundamentally different from that in isotropic dispersions. In the nematic, percolation depends only very weakly on the density and is, in essence, determined by a distance criterion that defines connectivity. This unexpected finding has its roots in the nontrivial coupling between the density and the degree of orientational order that dictate the mean number of particle contacts. Clusters in the nematic are much longer than wide, suggesting the use of nematics for nanocomposites with strongly anisotropic transport properties.
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http://dx.doi.org/10.1103/PhysRevLett.122.097801DOI Listing
March 2019

Macroscopic Model for Sessile Droplet Evaporation on a Flat Surface.

Langmuir 2018 10 8;34(41):12471-12481. Epub 2018 Oct 8.

Department of Applied Physics , Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven , The Netherlands.

Evaporation of sessile droplets on a flat surface involves a complex interplay between phase change, diffusion, advection, and surface forces. In an attempt to significantly reduce the complexity of the problem and to make it manageable, we propose a simple model hinged on a surface free-energy-based relaxation dynamics of the droplet shape, a diffusive evaporation model, and a contact line pinning mechanism governed by a yield stress. Our model reproduces the known dynamics of droplet shape relaxation and of droplet evaporation, both in the absence and in the presence of contact line pinning. We show that shape relaxation during evaporation significantly affects the lifetime of a drop. We find that the dependence of the evaporation time on the initial contact angle is a function of the competition between the shape relaxation and evaporation and is strongly affected by any contact line pinning.
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http://dx.doi.org/10.1021/acs.langmuir.8b02374DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6193248PMC
October 2018

Nanoscale insight into silk-like protein self-assembly: effect of design and number of repeat units.

Phys Biol 2018 09 12;15(6):066010. Epub 2018 Sep 12.

Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium. Faculteit Technische Natuurkunde, Technische Universiteit Eindhoven, Postbus 513, 5600 MB Eindhoven, Netherlands.

By means of replica exchange molecular dynamics simulations we investigate how the length of a silk-like, alternating diblock oligopeptide influences its secondary and quaternary structure. We carry out simulations for two protein sizes consisting of three and five blocks, and study the stability of a single protein, a dimer, a trimer and a tetramer. Initial configurations of our simulations are β-roll and β-sheet structures. We find that for the triblock the secondary and quaternary structures upto and including the tetramer are unstable: the proteins melt into random coil structures and the aggregates disassemble either completely or partially. We attribute this to the competition between conformational entropy of the proteins and the formation of hydrogen bonds and hydrophobic interactions between proteins. This is confirmed by our simulations on the pentablock proteins, where we find that, as the number of monomers in the aggregate increases, individual monomers form more hydrogen bonds whereas their solvent accessible surface area decreases. For the pentablock β-sheet protein, the monomer and the dimer melt as well, although for the β-roll protein only the monomer melts. For both trimers and tetramers remain stable. Apparently, for these the entropy loss of forming β-rolls and β-sheets is compensated for in the free-energy gain due to the hydrogen-bonding and hydrophobic interactions. We also find that the middle monomers in the trimers and tetramers are conformationally much more stable than the ones on the top and the bottom. Interestingly, the latter are more stable on the tetramer than on the trimer, suggesting that as the number of monomers increases protein-protein interactions cooperatively stabilize the assembly. According to our simulations, the β-roll and β-sheet aggregates must be approximately equally stable.
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http://dx.doi.org/10.1088/1478-3975/aadb5eDOI Listing
September 2018

Experimental and Theoretical Determination of the pH inside the Confinement of a Virus-Like Particle.

Small 2018 09 13;14(36):e1802081. Epub 2018 Aug 13.

Laboratory of Biomolecular Nanotechnology, MESA+ Institute for Nanotechnology, University of Twente, Enschede, 7500, AE, The Netherlands.

In biology, a variety of highly ordered nanometer-size protein cages is found. Such structures find increasing application in, for example, vaccination, drug delivery, and catalysis. Understanding the physiochemical properties, particularly inside the confinement of a protein cage, helps to predict the behavior and properties of new materials based on such particles. Here, the relation between the bulk solution pH and the local pH inside a model protein cage, based on virus-like particles (VLPs) built from the coat proteins of the cowpea chlorotic mottle virus, is investigated. The pH is a crucial parameter in a variety of processes and is potentially significantly influenced by the high concentration of charges residing on the interior of the VLPs. The data show a systematic more acidic pH of 0.5 unit inside the VLP compared to that of the bulk solution for pH values above pH 6, which is explained using a theoretical model based on a Donnan equilibrium. The model agrees with the experimental data over almost two orders of magnitude, while below pH 6 the experimental data point to a buffering capacity of the VLP. These results are a first step in a better understanding of the physiochemical conditions inside a protein cage.
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http://dx.doi.org/10.1002/smll.201802081DOI Listing
September 2018

Connectedness percolation of hard convex polygonal rods and platelets.

J Chem Phys 2018 Aug;149(5):054902

Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands.

The properties of polymer composites with nanofiller particles change drastically above a critical filler density known as the percolation threshold. Real nanofillers, such as graphene flakes and cellulose nanocrystals, are not idealized disks and rods but are often modeled as such. Here we investigate the effect of the shape of the particle cross section on the geometric percolation threshold. Using connectedness percolation theory and the second-virial approximation, we analytically calculate the percolation threshold of hard convex particles in terms of three single-particle measures. We apply this method to polygonal rods and platelets and find that the universal scaling of the percolation threshold is lowered by decreasing the number of sides of the particle cross section. This is caused by the increase of the surface area to volume ratio with decreasing number of sides.
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http://dx.doi.org/10.1063/1.5040185DOI Listing
August 2018

Hydrophobic-Interaction-Induced Stiffening of α-Synuclein Fibril Networks.

Phys Rev Lett 2018 May;120(20):208102

Nanobiophysics, MESA+ Institute for Nanotechnology and MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

In water, networks of semiflexible fibrils of the protein α-synuclein stiffen significantly with increasing temperature. We make plausible that this reversible stiffening is a result of hydrophobic contacts between the fibrils that become more prominent with increasing temperature. The good agreement of our experimentally observed temperature dependence of the storage modulus of the network with a scaling theory linking network elasticity with reversible cross-linking enables us to quantify the endothermic binding enthalpy and estimate the effective size of hydrophobic patches on the fibril surface. Our findings may not only shed light on the role of amyloid deposits in disease conditions, but can also inspire new approaches for the design of thermoresponsive materials.
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http://dx.doi.org/10.1103/PhysRevLett.120.208102DOI Listing
May 2018

The different faces of mass action in virus assembly.

J Biol Phys 2018 06 3;44(2):163-179. Epub 2018 Apr 3.

Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands.

The spontaneous encapsulation of genomic and non-genomic polyanions by coat proteins of simple icosahedral viruses is driven, in the first instance, by electrostatic interactions with polycationic RNA binding domains on these proteins. The efficiency with which the polyanions can be encapsulated in vitro, and presumably also in vivo, must in addition be governed by the loss of translational and mixing entropy associated with co-assembly, at least if this co-assembly constitutes a reversible process. These forms of entropy counteract the impact of attractive interactions between the constituents and hence they counteract complexation. By invoking mass action-type arguments and a simple model describing electrostatic interactions, we show how these forms of entropy might settle the competition between negatively charged polymers of different molecular weights for co-assembly with the coat proteins. In direct competition, mass action turns out to strongly work against the encapsulation of RNAs that are significantly shorter, which is typically the case for non-viral (host) RNAs. We also find that coat proteins favor forming virus particles over nonspecific binding to other proteins in the cytosol even if these are present in vast excess. Our results rationalize a number of recent in vitro co-assembly experiments showing that short polyanions are less effective at attracting virus coat proteins to form virus-like particles than long ones do, even if both are present at equal weight concentrations in the assembly mixture.
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http://dx.doi.org/10.1007/s10867-018-9487-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5928020PMC
June 2018

Continuum percolation of polydisperse rods in quadrupole fields: Theory and simulations.

J Chem Phys 2018 Jan;148(3):034903

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 3500 MB Eindhoven, The Netherlands.

We investigate percolation in mixtures of nanorods in the presence of external fields that align or disalign the particles with the field axis. Such conditions are found in the formulation and processing of nanocomposites, where the field may be electric, magnetic, or due to elongational flow. Our focus is on the effect of length polydispersity, which-in the absence of a field-is known to produce a percolation threshold that scales with the inverse weight average of the particle length. Using a model of non-interacting spherocylinders in conjunction with connectedness percolation theory, we show that a quadrupolar field always increases the percolation threshold and that the universal scaling with the inverse weight average no longer holds if the field couples to the particle length. Instead, the percolation threshold becomes a function of higher moments of the length distribution, where the order of the relevant moments crucially depends on the strength and type of field applied. The theoretical predictions compare well with the results of our Monte Carlo simulations, which eliminate finite size effects by exploiting the fact that the universal scaling of the wrapping probability function holds even in anisotropic systems. Theory and simulation demonstrate that the percolation threshold of a polydisperse mixture can be lower than that of the individual components, confirming recent work based on a mapping onto a Bethe lattice as well as earlier computer simulations involving dipole fields. Our work shows how the formulation of nanocomposites may be used to compensate for the adverse effects of aligning fields that are inevitable under practical manufacturing conditions.
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http://dx.doi.org/10.1063/1.5010979DOI Listing
January 2018

Impact of interaction range and curvature on crystal growth of particles confined to spherical surfaces.

Phys Rev E 2017 Jul 31;96(1-1):012611. Epub 2017 Jul 31.

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands and Instituut voor Theoretische Fysica, Universiteit Utrecht, The Netherlands.

When colloidal particles form a crystal phase on a spherical template, their packing is governed by the effective interaction between them and the elastic strain of bending the growing crystal. For example, if growth commences under appropriate conditions, and the isotropic crystal that forms reaches a critical size, growth continues via the incorporation of defects to alleviate elastic strain. Recently, it was experimentally found that, if defect formation is somehow not possible, the crystal instead continues growing in ribbons that protrude from the original crystal. Here we report on computer simulations in which we observe both the formation of ribbons at short interaction ranges and packings that incorporate defects if the interaction is longer-ranged. The ribbons only form above some critical crystal size, below which the nucleus is disk-shaped. We find that the scaling of the critical crystal size differs slightly from the one proposed in the literature, and we argue that this is because the actual morphology transition is caused by the competition between line tension and elastic stress, rather than the competition between chemical potential and elastic stress.
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http://dx.doi.org/10.1103/PhysRevE.96.012611DOI Listing
July 2017

Self-organisation of semi-flexible rod-like particles.

J Chem Phys 2017 Dec;147(24):244901

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

We report on a comprehensive computer simulation study of the liquid-crystal phase behaviour of purely repulsive, semi-flexible rod-like particles. For the four aspect ratios we consider, the particles form five distinct phases depending on their packing fraction and bending flexibility: the isotropic, nematic, smectic A, smectic B, and crystal phase. Upon increasing the particle bending flexibility, the various phase transitions shift to larger packing fractions. Increasing the aspect ratio achieves the opposite effect. We find two different ways in which the layer thickness of the particles in the smectic A phase may respond to an increase in concentration. The layer thickness may either decrease or increase depending on the aspect ratio and flexibility. For the smectic B and the crystalline phases, increasing the concentration always decreases the layer thickness. Finally, we find that the layer spacing jumps to a larger value on transitioning from the smectic A phase to the smectic B phase.
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http://dx.doi.org/10.1063/1.5000228DOI Listing
December 2017
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