Publications by authors named "Holger Merlitz"

35 Publications

Chain stiffness boosts active nanoparticle transport in polymer networks.

Phys Rev E 2021 May;103(5-1):052501

Departments of Mathematics, Applied Physical Sciences, Biomedical Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3250, USA.

Recent advances in technologies such as nanomanufacturing and nanorobotics have opened new pathways for the design of active nanoparticles (NPs) capable of penetrating biolayers for biomedical applications, e.g., for drug delivery. The coupling and feedback between active NP motility (with large stochastic increments relative to passive NPs) and the induced nonequilibrium deformation and relaxation responses of the polymer network, spanning scales from the NP to the local structure of the network, remain to be clarified. Using molecular dynamics simulations, combined with a Rouse mode analysis of network chains and position and velocity autocorrelation functions of the NPs, we demonstrate that the mobility of active NPs within cross-linked, concentrated polymer networks is a monotonically increasing function of chain stiffness, contrary to passive NPs, for which chain stiffness suppresses mobility. In flexible networks, active NPs exhibit a behavior similar to passive NPs, with a boost in mobility proportional to the self-propulsion force. These results are suggestive of design strategies for active NP penetration of stiff biopolymer matrices.
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http://dx.doi.org/10.1103/PhysRevE.103.052501DOI Listing
May 2021

Chemotaxis of Cargo-Carrying Self-Propelled Particles.

Phys Rev Lett 2021 May;126(20):208102

Leibniz-Institut für Polymerforschung Dresden, Institut Theory der Polymere, 01069 Dresden, Germany.

Active particles with their characteristic feature of self-propulsion are regarded as the simplest models for motility in living systems. The accumulation of active particles in low activity regions has led to the general belief that chemotaxis requires additional features and at least a minimal ability to process information and to control motion. We show that self-propelled particles display chemotaxis and move into regions of higher activity if the particles perform work on passive objects, or cargo, to which they are bound. The origin of this cooperative chemotaxis is the exploration of the activity gradient by the active particle when bound to a load, resulting in an average excess force on the load in the direction of higher activity. Using a new theoretical model, we capture the most relevant features of these active-passive dimers, and in particular we predict the crossover between antichemotactic and chemotactic behavior. Moreover, we show that merely connecting active particles to chains is sufficient to obtain the crossover from antichemotaxis to chemotaxis with increasing chain length. Such an active complex is capable of moving up a gradient of activity such as provided by a gradient of fuel and to accumulate where the fuel concentration is at its maximum. The observed transition is of significance to protoforms of life, enabling them to locate a source of nutrients even in the absence of any supporting sensomotoric apparatus.
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http://dx.doi.org/10.1103/PhysRevLett.126.208102DOI Listing
May 2021

FRET-Integrated Polymer Brushes for Spatially Resolved Sensing of Changes in Polymer Conformation.

Angew Chem Int Ed Engl 2021 07 17;60(30):16600-16606. Epub 2021 Jun 17.

Institute of Physical Chemistry and Polymer Physics, Leibniz-Institut für Polymerforschung e. V., Hohe Str. 6, 01069, Dresden, Germany.

Polymer brush surfaces that alter their physical properties in response to chemical stimuli have the capacity to be used as new surface-based sensing materials. For such surfaces, detecting the polymer conformation is key to their sensing capabilities. Herein, we report on FRET-integrated ultrathin (<70 nm) polymer brush surfaces that exhibit stimuli-dependent FRET with changing brush conformation. Poly(N-isopropylacrylamide) polymers were chosen due their exceptional sensitivity to liquid mixture compositions and their ability to be assembled into well-defined polymer brushes. The brush transitions were used to optically sense changes in liquid mixture compositions with high spatial resolution (tens of micrometers), where the FRET coupling allowed for noninvasive observation of brush transitions around complex interfaces with real-time sensing of the liquid environment. Our methods have the potential to be leveraged towards greater surface-based sensing capabilities at intricate interfaces.
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http://dx.doi.org/10.1002/anie.202104204DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8361709PMC
July 2021

Pseudo-chemotaxis of active Brownian particles competing for food.

PLoS One 2020 8;15(4):e0230873. Epub 2020 Apr 8.

Institut Theorie der Polymere, Leibniz-Institut für Polymerforschung Dresden, Dresden, Germany.

Active Brownian particles (ABPs) are physical models for motility in simple life forms and easily studied in simulations. An open question is to what extent an increase of activity by a gradient of fuel, or food in living systems, results in an evolutionary advantage of actively moving systems such as ABPs over non-motile systems, which rely on thermal diffusion only. It is an established fact that within confined systems in a stationary state, the activity of ABPs generates density profiles that are enhanced in regions of low activity, which is thus referred to as 'anti-chemotaxis'. This would suggest that a rather complex sensoric subsystem and information processing is a precondition to recognize and navigate towards a food source. We demonstrate in this work that in non-stationary setups, for instance as a result of short bursts of fuel/food, ABPs do in fact exhibit chemotactic behavior. In direct competition with inactive, but otherwise identical Brownian particles (BPs), the ABPs are shown to fetch a larger amount of food. We discuss this result based on simple physical arguments. From the biological perspective, the ability of primitive entities to move in direct response to the available amount of external energy would, even in absence of any sensoric devices, encompass an evolutionary advantage.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0230873PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7141648PMC
July 2020

Mechanical Strength Management of Polymer Composites through Tuning Transient Networks.

J Phys Chem Lett 2020 Feb 15;11(3):710-715. Epub 2020 Jan 15.

Department of Physics , Xiamen University , Xiamen 361005 , People's Republic of China.

The addition of transient networks to polymer composites marks a new direction toward the design of novel materials, with numerous biomedical and industrial applications. The network structure connected by transient cross-links (CLs) relaxes as time evolves, which results in the stretching release of polymer strands between transient CLs during strain. Using molecular dynamics simulations, we measure directly the stress-strain curves of double polymer networks (DPNs), containing both transient and permanent components, at different strain rates. Lifetime and density of transient CLs control the relaxation spectrum of transient networks and determine the mechanical properties of DPNs. A Rouse mode analysis reveals that at high strain rates the mechanical strength of DPNs is defined jointly by the cross-linking structures of permanent and transient networks. At low strain rates, the cross-linking structure of transient network relaxes, leaving the permanent component of the network as a sole contributor to the mechanical strength of DPNs. The transient network is shown to facilitate a dissipation of energy at higher strain rates and prevents a rupture of the network, while the permanent network preserves the structural integrity of the composite at low strain rates. This study provides computational and theoretical foundations for designing polymer composites with desirable mechanical strength and toughness by means of tuning transient networks.
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http://dx.doi.org/10.1021/acs.jpclett.9b03697DOI Listing
February 2020

Nanoparticle Loading of Unentangled Polymers Induces Entanglement-Like Relaxation Modes and a Broad Sol-Gel Transition.

J Phys Chem Lett 2019 Sep 14;10(17):4968-4973. Epub 2019 Aug 14.

Departments of Mathematics and Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3250, United States.

We combine molecular dynamics simulations, imaging and data analysis, and the Green-Kubo summation formula for the relaxation modulus () to elicit the structure and rheology of unentangled polymer-nanoparticle composites distinguished by small NPs and strong NP-monomer attraction, ε ≫ . A reptation-like plateau emerges in () beyond a terminal relaxation time scale as the volume fraction, , of NPs increases, coincident with a structure transition. A condensed phase of NP-aggregates forms, tightly interlaced with thin sheets of polymer chains, the remaining phase consisting of free chains void of NPs. Rouse mode analyses are applied to the two individual phases, revealing that long-wavelength Rouse modes in the aggregate phase are the source of reptation-like relaxation. Imaging reveals chain motion confined within the thin sheets between NPs and exhibits a 2D analogue of classical reptation. In the NP-free phase, Rouse modes relax indistinguishable from a neat polymer melt. The Fourier transform of () reveals a sol-gel transition across a broad frequency spectrum, tuned by and ε above critical thresholds, below which all structure and rheological transitions vanish.
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http://dx.doi.org/10.1021/acs.jpclett.9b01954DOI Listing
September 2019

Linear response approach to active Brownian particles in time-varying activity fields.

J Chem Phys 2018 May;148(19):194116

Leibniz-Institut für Polymerforschung Dresden, Institut Theorie der Polymere, 01069 Dresden, Germany.

In a theoretical and simulation study, active Brownian particles (ABPs) in three-dimensional bulk systems are exposed to time-varying sinusoidal activity waves that are running through the system. A linear response (Green-Kubo) formalism is applied to derive fully analytical expressions for the torque-free polarization profiles of non-interacting particles. The activity waves induce fluxes that strongly depend on the particle size and may be employed to de-mix mixtures of ABPs or to drive the particles into selected areas of the system. Three-dimensional Langevin dynamics simulations are carried out to verify the accuracy of the linear response formalism, which is shown to work best when the particles are small (i.e., highly Brownian) or operating at low activity levels.
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http://dx.doi.org/10.1063/1.5025760DOI Listing
May 2018

Design of binary polymer brushes with tuneable functionality.

Soft Matter 2018 Sep;14(35):7237-7245

Physical Chemistry and Soft Matter, Wageningen University & Research, 6708 WE Wageningen, The Netherlands.

Using coarse grained molecular dynamics simulations, we study how functionalized binary brushes may be used to create surfaces whose functionality can be tuned. Our model brushes consist of a mixture of nonresponsive polymers with functionalized responsive polymers. The functional groups switch from an exposed to a hidden state when the conformations of the responsive polymers change from extended to collapsed. We investigate quantitatively which sets of brush parameters result in optimal switching in functionality, by analyzing to which extent the brush conformation allows an external object to interact with the functional groups. It is demonstrated that brushes with species of comparable polymer lengths, or with longer responsive polymers than nonresponsive polymers, can show significant differences in their functionality. In the latter case, either the fraction of responsive polymers or the total grafting density has to be reduced. Among these possibilities, a reduction of the fraction of responsive polymers is shown to be most effective.
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http://dx.doi.org/10.1039/c8sm01108gDOI Listing
September 2018

Pseudochemotaxis in inhomogeneous active Brownian systems.

Phys Rev E 2018 Apr;97(4-1):042612

Leibniz-Institut für Polymerforschung Dresden, Institut Theorie der Polymere, 01069 Dresden, Germany.

We study dynamical properties of confined, self-propelled Brownian particles in an inhomogeneous activity profile. Using Brownian dynamics simulations, we calculate the probability to reach a fixed target and the mean first passage time to the target of an active particle. We show that both these quantities are strongly influenced by the inhomogeneous activity. When the activity is distributed such that high-activity zone is located between the target and the starting location, the target finding probability is increased and the passage time is decreased in comparison to a uniformly active system. Moreover, for a continuously distributed profile, the activity gradient results in a drift of active particle up the gradient bearing resemblance to chemotaxis. Integrating out the orientational degrees of freedom, we derive an approximate Fokker-Planck equation and show that the theoretical predictions are in very good agreement with the Brownian dynamics simulations.
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http://dx.doi.org/10.1103/PhysRevE.97.042612DOI Listing
April 2018

Nanoparticles of Various Degrees of Hydrophobicity Interacting with Lipid Membranes.

J Phys Chem Lett 2017 Sep 16;8(17):4069-4076. Epub 2017 Aug 16.

Leibniz-Institut für Polymerforschung Dresden , 01069 Dresden, Germany.

Using coarse-grained molecular dynamics simulations, we study the passive translocation of nanoparticles with a size of about 1 nm and with tunable degrees of hydrophobicity through lipid bilayer membranes. We observe a window of translocation with a sharp maximum for nanoparticles having a hydrophobicity in between hydrophilic and hydrophobic. Passive translocation can be identified as diffusive motion of individual particles in a free energy landscape. By combining direct sampling with umbrella-sampling techniques we calculate the free energy landscape for nanoparticles covering a wide range of hydrophobicities. We show that the directly observed translocation rate of the nanoparticles can be mapped to the mean-escape-rate through the calculated free energy landscape, and the maximum of translocation can be related with the maximally flat free energy landscape. The limiting factor for the translocation rate of nanoparticles having an optimal hydrophobicity can be related with a trapping of the particles in the surface region of the membrane. Here, hydrophobic contacts can be formed but the free energy effort of insertion into the brush-like tail regions can still be avoided. The latter forms a remaining barrier of a few kT and can be spontaneously surmounted. We further investigate cooperative effects of a larger number of nanoparticles and their impact on the membrane properties such as solvent permeability, area per lipid, and the orientation order of the tails. By calculating the partition of nanoparticles at the phase boundary between water and oil, we map the microscopic parameter of nanoparticle hydrophobicity to an experimentally accessibly partition coefficient. Our studies reveal a generic mechanism for spherical nanoparticles to overcome biological membrane-barriers without the need of biologically activated processes.
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http://dx.doi.org/10.1021/acs.jpclett.7b01888DOI Listing
September 2017

Tuning Adsorption Duration To Control the Diffusion of a Nanoparticle in Adsorbing Polymers.

J Phys Chem Lett 2017 Jun 31;8(12):2629-2633. Epub 2017 May 31.

Department of Physics, Xiamen University , Xiamen 361005, P. R. China.

Controlling the nanoparticle (NP) diffusion in polymers is a prerequisite to obtain polymer nanocomposites (PNCs) with desired dynamical and rheological properties and to achieve targeted delivery of nanomedicine in biological systems. Here we determine the suppression mechanism of direct NP-polymer attraction to hamper the NP mobility in adsorbing polymers and then quantify the dependence of the effective viscosity η felt by the NP on the adsorption duration τ of polymers on the NP using scaling theory analysis and molecular dynamics simulations. We propose and confirm that participation of adsorbed chains in the NP motion break up at time intervals beyond τ due to the rearrangement of polymer segments at the NP surface, which accounts for the onset of Fickian NP diffusion on a time scale of t ≈ τ. We develop a power law, η ∼ (τ), where ν is the scaling exponent of the dependence of polymer coil size on the chain length, which leads to a theoretical basis for the design of PNCs and nanomedicine with desired applications through tuning the polymer adsorption duration.
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http://dx.doi.org/10.1021/acs.jpclett.7b01049DOI Listing
June 2017

Directional transport of colloids inside a bath of self-propelling walkers.

Soft Matter 2017 May;13(20):3726-3733

Leibniz-Institut für Polymerforschung Dresden, 01069 Dresden, Germany.

We present a setup in which passive colloids inside a solvent are moved to the boundaries of the container. The directional transport is facilitated by self-propelling microparticles ("walkers") with an activity gradient, which reduces their propulsion in the vicinity of bounding walls. An attractive interaction leads to the adsorption of walkers onto the colloid-surfaces in regions of low walker activity. It is shown that the activity gradient generates a free energy gradient which in turn acts as a driving force on the passive colloids. We carry out molecular dynamics simulations and present approaches to a theoretical description of the involved processes. Although the simulation data are not reproduced on a fully quantitative level, their qualitative features are covered by the model. The effect described here may be applied to facilitate a directional transport of drugs or to eliminate pollutants.
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http://dx.doi.org/10.1039/c7sm00613fDOI Listing
May 2017

Mixed brush made of 4-arm stars and linear chains: MD simulations.

J Chem Phys 2016 Dec;145(23):234905

Leibniz-Institut für Polymerforschung Dresden, 01069 Dresden, Germany.

We investigate the structural properties of binary polymer brushes, composed of functional 4-armed star polymers and chemically identical linear polymers of different molecular weights. The molecular dynamics simulations confirm recent self-consistent field studies, in which a considerable potential of these systems for the design of switchable surfaces has been claimed. The length of the linear chains serves as a control parameter, which, while passing over a critical value, induces a sharp transition of the molecular conformation. We investigate these transitions at different grafting densities and summarize our findings in a phase diagram. The temperature dependence of the brush structure is investigated in a non-selective solvent, and non-trivial variations of the surface composition are observed. The quantity of these latter effects would be insufficient to build switchable systems, and we argue that a minor quantity of solvent selectivity would suffice to enable the desired feature of an environment-responsive coating.
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http://dx.doi.org/10.1063/1.4971379DOI Listing
December 2016

In Vitro Assembly Kinetics of Cytoplasmic Intermediate Filaments: A Correlative Monte Carlo Simulation Study.

PLoS One 2016 15;11(6):e0157451. Epub 2016 Jun 15.

Institute of Neuropathology, University Hospital Erlangen, Erlangen, Germany.

Intermediate filament (IF) elongation proceeds via full-width "mini-filaments", referred to as "unit-length" filaments (ULFs), which instantaneously form by lateral association of extended coiled-coil complexes after assembly is initiated. In a comparatively much slower process, ULFs longitudinally interact end-to-end with other ULFs to form short filaments, which further anneal with ULFs and with each other to increasingly longer filaments. This assembly concept was derived from time-lapse electron and atomic force microscopy data. We previously have quantitatively verified this concept through the generation of time-dependent filament length-profiles and an analytical model that describes assembly kinetics well for about the first ten minutes. In this time frame, filaments are shorter than one persistence length, i.e. ~1 μm, and thus filaments were treated as stiff rods associating via their ends. However, when filaments grow several μm in length over hours, their flexibility becomes a significant factor for the kinetics of the longitudinal annealing process. Incorporating now additional filament length distributions that we have recorded after extended assembly times by total internal reflection fluorescence microscopy (TIRFM), we developed a Monte Carlo simulation procedure that accurately describes the underlying assembly kinetics for large time scales.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0157451PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4909217PMC
July 2017

A theoretical study of dispersion-to-aggregation of nanoparticles in adsorbing polymers using molecular dynamics simulations.

Nanoscale 2016 Apr;8(13):6964-8

Leibniz-Institut für Polymerforschung Dresden, 01069 Dresden, Germany and Technische Universität Dresden, Institute of Theoretical Physics, D-01069 Dresden, Germany.

The properties of polymer-nanoparticle (NP) mixtures significantly depend on the dispersion of the NPs. Using molecular dynamics simulations, we demonstrate that, in the presence of polymer-NP attraction, the dispersion of NPs in semidilute and concentrated polymers can be stabilized by increasing the polymer concentration. A lower polymer concentration facilitates the aggregation of NPs bridged by polymer chains, as well as a further increase of the polymer-NP attraction. Evaluating the binding of NPs through shared polymer segments in an adsorption blob, we derive a linear relationship between the polymer concentration and the polymer-NP attraction at the phase boundary between dispersed and aggregated NPs. Our theoretical findings are directly relevant for understanding and controlling many self-assembly processes that use either dispersion or aggregation of NPs to yield the desired materials.
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http://dx.doi.org/10.1039/c5nr08576dDOI Listing
April 2016

Non-synchronization of lattice and carrier temperatures in light-emitting diodes.

Sci Rep 2016 Jan 20;6:19539. Epub 2016 Jan 20.

Department of Electronic Science, Fujian Engineering Research Center for Solid-state Lighting, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China.

Pulse implementation or switching-off (PISO) of electrical currents has become a common operation in junction-temperature (Tj) measurements for semiconductor devices since 2004. Here we have experimentally discovered a substantial discrepancy between Tj values with, and without, PISO (e.g., 36.8 °C versus 76.5 °C above the ambient temperature at 25.0 °C). Our research indicates that methods associated with PISO are flawed due to non-synchronization of lattice temperatures and carrier temperatures in transient states. To scrutinize this discrepancy, we propose a lattice-inertia thermal anchoring mechanism that (1) explains the cause of this discrepancy, (2) helps to develop a remedy to eliminate this discrepancy by identifying three transient phases, (3) has been applied to establishing an original, accurate, and noninvasive technique for light-emitting diodes to measure Tj in the absence of PISO. Our finding may pave the foundation for LED communities to further establish reliable junction-temperature measurements based on the identified mechanism.
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http://dx.doi.org/10.1038/srep19539DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4726174PMC
January 2016

Counterion-mediated protein adsorption into polyelectrolyte brushes.

Eur Phys J E Soft Matter 2015 Sep 21;38(9):101. Epub 2015 Sep 21.

Department of Physics and ITPA, Xiamen University, 361005, Xiamen, P.R. China.

We present molecular dynamics simulations of the interaction of fullerene-like, inhomogeneously charged proteins with polyelectrolyte brushes. A motivation of this work is the experimental observation that proteins, carrying an integral charge, may enter like-charged polymer brushes. Simulations of varying charge distributions on the protein surfaces are performed to unravel the physical mechanism of the adsorption. Our results prove that an overall neutral protein can be strongly driven into polyelectrolyte brush whenever the protein features patches of positive and negative charge. The findings reported here give further evidence that the strong adsorption of proteins is also driven by entropic forces due to counterion release, since charged patches on the surface of the proteins can act as multivalent counterions of the oppositely charged polyelectrolyte chains. A corresponding number of mobile co- and counterions is released from the brush and the vicinity of the proteins so that the entropy of the total system increases.
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http://dx.doi.org/10.1140/epje/i2015-15101-9DOI Listing
September 2015

Performance of binoculars: Berek's model of target detection.

Authors:
Holger Merlitz

J Opt Soc Am A Opt Image Sci Vis 2015 Jan;32(1):101-7

A model of target detection thresholds, first presented by Max Berek of Leitz, is fitted into a simple set of closed equations. These are combined with a recently published universal formula for the human eye's pupil size to yield a versatile formalism that is capable of predicting binocular performance gains. The model encompasses target size, contrast, environmental luminance, binocular's objective diameter, magnification, angle of view, transmission, stray light, and the observer's age. We analyze performance parameters of various common binocular models and compare the results with popular approximations to binocular performance, like the well-known twilight index. The formalisms presented here are of interest in military target detection as well as in civil applications such as hunting, surveillance, object security, law enforcement, and astronomy.
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http://dx.doi.org/10.1364/JOSAA.32.000101DOI Listing
January 2015

Maximal rectification ratios for idealized bi-segment thermal rectifiers.

Sci Rep 2015 Aug 4;5:12677. Epub 2015 Aug 4.

Department of Electronic Science, Fujian Engineering Research Center for Solid-state Lighting, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, China 361005.

Thermal rectifiers whose forward heat fluxes are greater than reverse counterparts have been extensively studied. Here we have discovered, idealized, and derived the ultimate limit of such rectification ratios, which are partially validated by numerical simulations, experiments, and micro-scale Hamiltonian-oscillator analyses. For rectifiers whose thermal conductivities (κ) are linear with the temperature, this limit is simply a numerical value of 3. For those whose conductivities are nonlinear with temperatures, the maxima equal κmax/κmin, where two extremes denote values of the solid segment materials that can be possibly found or fabricated within a reasonable temperature range. Recommendations for manufacturing high-ratio rectifiers are also given with examples. Under idealized assumptions, these proposed rectification limits cannot be defied by any bi-segment thermal rectifiers.
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http://dx.doi.org/10.1038/srep12677DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4523837PMC
August 2015

Polyelectrolyte brushes in external fields: molecular dynamics simulations and mean-field theory.

Soft Matter 2015 Jul;11(28):5688-96

Department of Physics and ITPA, Xiamen University, Xiamen 361005, P. R. China.

A mean-field model is developed to predict the layer-thickness of sparse and salt-free polyelectrolyte brushes, exposed to an external electric field, which attracts the polyelectrolytes to the substrate. In molecular dynamics simulations, it is shown that a fraction of polymers collapses entirely to screen the charge of the substrate. The remaining brushes are then treated as field-free brushes at reduced grafting density. The mean-field model may thus be applied to field-free brushes, both in their osmotic and their weak charge regimes. It yields simple, closed equations for the brush height and for the partition of counterions in- and outside the brushes, and accurately reproduces simulation data of the collapse of the brushes during the crossover between both charge regimes.
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http://dx.doi.org/10.1039/c5sm01275aDOI Listing
July 2015

Molecular dynamics simulations of polyelectrolyte brushes under poor solvent conditions: origins of bundle formation.

J Chem Phys 2014 Mar;140(10):104911

Leibniz-Institut für Polymerforschung Dresden, 01069 Dresden, Germany.

Molecular dynamics simulations are applied to investigate salt-free planar polyelectrolyte brushes under poor solvent conditions. Starting above the Θ-point with a homogeneous brush and then gradually reducing the temperature, the polymers initially display a lateral structure formation, forming vertical bundles of chains. A further reduction of the temperature (or solvent quality) leads to a vertical collapse of the brush. By varying the size and selectivity of the counterions, we show that lateral structure formation persists and therefore demonstrate that the entropy of counterions being the dominant factor for the formation of the bundle phase. By applying an external compression force on the brush we calculate the minimal work done on the polymer phase only and prove that the entropy gain of counterions in the bundle state, as compared to the homogeneously collapsed state at the same temperature, is responsible for the lateral microphase segregation. As a consequence, the observed lateral structure formation has to be regarded universal for osmotic polymer brushes below the Θ-point.
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http://dx.doi.org/10.1063/1.4867466DOI Listing
March 2014

Numerical evidences for a free energy barrier in starlike polymer brushes.

J Chem Phys 2013 Oct;139(13):134910

Department of Physics and ITPA, Xiamen University, Xiamen 361005, People's Republic of China.

The existence of a free energy barrier, which prohibits the upward motion of retracted molecules into the surface region of starlike polymer brushes, is analyzed through molecular dynamics simulations in good solvent. This barrier emerges at moderate and high grafting densities, as a result of a density-discontinuity at the branching points of the highly stretched starlike molecules. The vertical force profiles of brushes of varying densities are taken with the help of a probe-particle that is gradually moved into the brush, and the results are compared with the density profiles and their negative gradients which generate the local osmotic pressures. Chain expulsion simulations, supported by scaling theory, are conducted to understand the dynamics of individual molecules inside the brushes. We prove that the flip-rates between retracted and extended states, being of relevance for the generation of efficiently switchable, environment-responsive brush layers, are determined by the elastic tension of the stretched molecules.
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http://dx.doi.org/10.1063/1.4823766DOI Listing
October 2013

Polymer-induced inverse-temperature crystallization of nanoparticles on a substrate.

ACS Nano 2013 Nov 7;7(11):9920-6. Epub 2013 Oct 7.

Leibniz-Institut für Polymerforschung Dresden, 01069 Dresden, Germany.

Using molecular dynamics simulations, we study the properties of liquid state polymer-nanoparticle composites confined between two parallel substrates, with an attractive polymer-substrate interaction. Polymers are in the semidilute regime at concentrations far above the overlap point, and nanoparticles are in good solvent and without enthalpic attraction to the substrates. An increase of temperature then triggers the crystallization of nanoparticles on one of the two substrate surfaces-a surprising phenomenon, which is explained in terms of scaling theory, such as through competing effects of adsorption-and correlation blobs. Moreover, we show that the first, closely packed layer of nanoparticles on the substrate increases the depletion attraction of additional nanoparticles from the bulk, thereby enhancing and stabilizing the formation of a crystalline phase on the substrate. Within the time frame accessible to our numerical simulations, the crystallization of nanoparticles was irreversible; that is, their crystalline phase, once created, remained undamaged after a decrease of the temperature. Our study leads to a class of thermoreactive nanomaterials, in which the transition between a homogeneous state with dissolved nanoparticles and a surface-crystallized state is triggered by a temperature jump.
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http://dx.doi.org/10.1021/nn4037738DOI Listing
November 2013

Polymer-induced entropic depletion potential.

Phys Rev E Stat Nonlin Soft Matter Phys 2011 Oct 12;84(4 Pt 1):041802. Epub 2011 Oct 12.

Department of Physics and ITPA, Xiamen University, Xiamen 361005, People's Republic of China.

We study the effective interactions between nanoparticles immersed in an athermal polymer solution using Molecular dynamics. The directly measured polymer-induced depletion forces are well described with a scaling model in which the attraction between particles is caused by the depletion of concentration blobs and thus independent of the length of the polymer chains. We find strong evidence for a repulsive barrier which arises when the distance between the particles is of the order of the correlation length of the solution and which can be interpreted as a packing effect of concentration blobs. Interestingly, the scaling picture can be extended into the regime in which higher virial coefficients of the polymer solution become relevant. We derive a universal relation between the attraction force at the particle contact, f(0), and the osmotic pressure Π as f(0)∼Π(2/3), demonstrating its validity over a wide range of concentrations of the polymer solution.
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http://dx.doi.org/10.1103/PhysRevE.84.041802DOI Listing
October 2011

Distortion of binoculars revisited: does the sweet spot exist?

Authors:
Holger Merlitz

J Opt Soc Am A Opt Image Sci Vis 2010 Jan;27(1):50-7

Department of Physics and ITPA, Xiamen University, Xiamen 361005, China.

Sixty years ago, August Sonnefeld of Zeiss reported on observations with experimental telescopes. The goal of his investigation was to determine the ideal amount of distortion applied to optical instruments that are used in combination with the human eye. His studies were inconclusive and partially contradictory. We have picked up this problem once again, adopting a modern point of view about the human imaging process, and supported by computer graphics. Based on experiments with Helmholtz checkerboards, we argue that human imaging introduces a certain amount of barrel distortion, which has to be counterbalanced through the implementation of an equally strong pincushion distortion into the binocular design. We discuss in detail how this approach is capable of eliminating the globe effect of the panning binocular and how the residual pincushion distortion affects the image when the eye is pointing off-center. Our results support the binocular designer in optimizing his instrument for its intended mode of application, and may help binocular users and astronomers better understand their tools.
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http://dx.doi.org/10.1364/JOSAA.27.000050DOI Listing
January 2010

How proteins squeeze through polymer networks: a Cartesian lattice study.

J Chem Phys 2009 Aug;131(6):064905

BIOMS Center for Modeling and Simulation in the Biosciences, D-69120 Heidelberg, Germany.

In this paper a lattice model for the diffusional transport of particles in the interphase cell nucleus is proposed. The dynamical behavior of single chains on the lattice is investigated and Rouse scaling is verified. Dynamical dense networks are created by a combined version of the bond fluctuation method and a Metropolis Monte Carlo algorithm. Semidilute behavior of the dense chain networks is shown. By comparing diffusion of particles in a static and a dynamical chain network, we demonstrate that chain diffusion does not alter the diffusion process of small particles. However, we prove that a dynamical network facilitates the transport of large particles. By weighting the mean square displacement trajectories of particles in the static chain, network data from the dynamical network can be reconstructed. Additionally, it is shown that subdiffusive behavior of particles on short time scales results from trapping processes in the crowded environment of the chain network. In the presented model a protein with 30 nm diameter has an effective diffusion coefficient of 1.24 x 10(-11) m2/s in a chromatin fiber network.
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http://dx.doi.org/10.1063/1.3205100DOI Listing
August 2009

Polymer brushes for surface tuning.

Macromol Rapid Commun 2009 May 13;30(9-10):732-40. Epub 2009 May 13.

Leibniz-Institut für Polymerforschung Dresden, 01069 Dresden, Germany.

Mixed polymer brushes as functional ultra thin films for surface functionalization have an enormous potential to create a variety of smart, switchable, and multifunctional surfaces and thin films. It is shown how computer simulations can contribute to a better understanding of the switching behavior of brushes. Furthermore, it is described how polymer brushes can be used to create surfaces with switchable ultrahydrophobicity and wettability gradients, as well as functional layers for the immobilization of nanoparticles. Applications of these versatile and multifunctional brush coatings are envisioned in many areas including fluid control, microfluidics, and thin film sensors.
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http://dx.doi.org/10.1002/marc.200900113DOI Listing
May 2009

Nanoscale brushes: how to build a smart surface coating.

Phys Rev Lett 2009 Mar 20;102(11):115702. Epub 2009 Mar 20.

Department of Physics, Xiamen University, Xiamen 361005, People's Republic of China.

Via computer simulations, we demonstrate how a densely grafted layer of polymers, a brush, could be turned into an efficient switch through chemical modification of some of its end monomers. In this way, a surface coating with reversibly switchable properties can be constructed. We analyze the fundamental physical principle behind its function, a recently discovered surface instability, and demonstrate that the combination of a high grafting density, an inflated end-group size, and a high degree of monodispersity is a condition for an optimal functionality of the switch.
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http://dx.doi.org/10.1103/PhysRevLett.102.115702DOI Listing
March 2009

The role of chromatin conformations in diffusional transport of chromatin-binding proteins: Cartesian lattice simulations.

J Chem Phys 2008 Apr;128(15):155101

Deutsches Krebsforschungszentrum, Heidelberg, Germany.

In this paper, a lattice model for the diffusional transport of chromatin-binding particles in the interphase cell nucleus is proposed. Sliding effects are studied in dense networks of chromatin fibers created by three different methods: Randomly distributed, noninterconnected obstacles, a random walk chain model with an attractive step potential, and a self-avoiding random walk chain model with a hard repulsive core and attractive surroundings. By comparing a discrete and continuous version of the random walk chain model, we demonstrate that lattice discretization does not alter the diffusion of chromatin-binding particles. The influence of conformational properties of the fiber network on the particle sliding is investigated in detail while varying occupation volume, sliding probability, chain length, and persistence length. It is observed that adjacency of the monomers, the excluded volume effect incorporated in the self-avoiding random walk model, and the persistence length affect the chromatin-binding particle diffusion. It is demonstrated that sliding particles sense local chain structures. When plotting the diffusion coefficient as a function of the accessible volume for diffusing particles, the data fall onto master curves depending on the persistence length. However, once intersegment transfer is involved, chromatin-binding proteins no longer perceive local chain structures.
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http://dx.doi.org/10.1063/1.2895048DOI Listing
April 2008

Modeling diffusional transport in the interphase cell nucleus.

J Chem Phys 2007 Jul;127(4):045102

Deutsches Krebsforschungszentrum, D-69120 Heidelberg, Germany.

In this paper a lattice model for the diffusional transport of particles in the interphase cell nucleus is proposed. Dense networks of chromatin fibers are created by three different methods: Randomly distributed, noninterconnected obstacles, a random walk chain model, and a self-avoiding random walk chain model with persistence length. By comparing a discrete and a continuous version of the random walk chain model, we demonstrate that lattice discretization does not alter particle diffusion. The influence of the three dimensional geometry of the fiber network on the particle diffusion is investigated in detail while varying the occupation volume, chain length, persistence length, and walker size. It is shown that adjacency of the monomers, the excluded volume effect incorporated in the self-avoiding random walk model, and, to a lesser extent, the persistence length affect particle diffusion. It is demonstrated how the introduction of the effective chain occupancy, which is a convolution of the geometric chain volume with the walker size, eliminates the conformational effects of the network on the diffusion, i.e., when plotting the diffusion coefficient as a function of the effective chain volume, the data fall onto a master curve.
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http://dx.doi.org/10.1063/1.2753158DOI Listing
July 2007
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