arXiv subjects

Andrey Milchev

Publications and source records attributed to Andrey Milchev.

At least 19 recordsLinked to original sources

arXiv2026

Directed droplet motion -- Its versatile nature and anticipated applications

Applications such as digital microfluidics and bio-diagnostics rely on droplet locomotion. A prominent example of such motion is durotaxis, a phenomenon that requires a stiffness gradient along a surface for the transport of liquids, cells, or other nano-objects. Using surfaces with varying properties in specific directions can be exploited as a universal concept for fluid transport with or without external energy supply. Changes in properties may refer to substrate patterns, Laplace pressure changes, wettability gradients, etc., leading to exciting phenomena, which can be employed in novel applications in various technologies. Here, we report on key results and progress in the area of directed droplet motion over the years, and we provide perspectives and implications for anticipated applications.

physics.flu-dyn↗

arXiv2024

Effect of simple shear on knotted polymer coils and globules

We explore the effect of Couette flow on knotted linear polymer chains with extensive Molecular Dynamics (MD) simulations. Hydrodynamic interactions are accounted for by means of Multi-Particle Collision Dynamics (MPCD). The polymer chain, containing originally a simple trefoil knot at rest, is described by a coarse-grained bead-spring model in a coil or globular state. We demonstrate that under shear existing loosely localized knots in polymer coils typically tighten to several segments beyond a certain shear rate threshold. At large shear rates the polymer undergoes a tumbling-like motion during which knot sizes can fluctuate. In contrast, sheared knotted globules unwind into a convoluted pearl-necklace structure of sub-globules that folds back onto itself and in which knot types change over time.

cond-mat.soft↗

arXiv2023

Antidurotaxis Droplet Motion onto Gradient Brush Substrates

Durotaxis motion is a spectacular phenomenon manifesting itself by the autonomous motion of a nano-object between parts of a substrate with different stiffness. This motion usually takes place along a stiffness gradient from softer to stiffer parts of the substrate. Here, we propose a new design of a polymer brush substrate that demonstrates antidurotaxis droplet motion, that is, droplet motion from stiffer to softer parts of the substrate. By carrying out extensive molecular dynamics simulation of a coarse-grained model, we find that antidurotaxis is solely controlled by the gradient in the grafting density of the brush and is favorable for fluids with a strong attraction to the substrate (low surface energy). The driving force of the antidurotaxial motion is the minimization of the droplet-substrate interfacial energy, which is attributed to the penetration of the droplet into the brush. Thus, we anticipate that the proposed substrate design offers a new understanding and possibilities in the area of autonomous motion of droplets for applications in microfluidics, energy conservation, and biology.

physics.flu-dyn↗

arXiv2023

Rugotaxis: Droplet motion without external energy supply

Nano-patterned substrates offer possibilities for controlling the motion of fluids without external energy supply in novel technologies in microfluidics, coatings, etc. Here, we report on the rugotaxial motion of droplets on wrinkled substrates with gradient in the wavelength of the wrinkles by exploring a broad range of parameters, such as amplitude of the wrinkles, substrate wettability, droplet size and wavelength gradient. Adopting a theoretical and molecular dynamics approach, we determine the Cassie-Baxter and Wenzel states of the droplets, investigate the efficiency of rugotaxis as a function of different parameters, and discuss additional effects, such as pinning. We find that shallow wrinkles characterised by small wavelength gradients, and moderate adhesion of the droplet to the substrate favour the rugotaxis motion with growing droplet size, when pinning is avoided. We also find that the driving force in rugotaxis is the gain in interfacial energy between the droplet and the substrate as the droplet enters regions of denser wrinkles (smaller wavelengths of the wrinkles).

physics.flu-dyn↗

arXiv2023

Unidirectional Droplet Propulsion onto Gradient Brushes without External Energy Supply

Using extensive molecular dynamics simulation of a coarse-grained model, we demonstrate the possibility of sustained unidirectional motion (durotaxis) of droplets without external energy supply when placed on a polymer brush substrate with stiffness gradient in a certain direction. The governing key parameters for the specific substrate design studied, which determine the durotaxis efficiency, are found to be the grafting density of the brush and the droplet adhesion to the brush surface, whereas the strength of the stiffness gradient, the viscosity of the droplet, or the length of the polymer chains of the brush have only a minor effect on the process. It is shown that this durotaxial motion is driven by the steady increase of the interfacial energy between droplet and brush as the droplet moves from softer to stiffer parts of the substrate whereby the mean driving force gradually declines with decreasing roughness of the brush surface. We anticipate that our findings indicate further possibilities in the area of nanoscale motion without external energy supply.

physics.flu-dyn↗

arXiv2020

Entropic Unmixing in Nematic Blends of Semiflexible Polymers

Binary mixtures of semiflexible polymers with the same chain length but different persistence lengths separate into two coexisting different nematic phases when the osmotic pressure of the lyotropic solution is varied. Molecular Dynamics simulations and Density Functional Theory predict phase diagrams either with a triple point, where the isotropic phase coexists with two nematic phases, or a critical point of unmixing within the nematic mixture. The difference in locally preferred bond angles between the constituents drives this unmixing without any attractive interactions between monomers.

cond-mat.soft↗

arXiv2019

Linear Dimensions of Adsorbed Semiflexible Polymers: What can be learned about their persistence length?

Conformations of partially or fully adsorbed semiflexible polymer chains are studied varying both contour length $L$, chain stiffness, $\kappa$, and the strength of the adsorption potential over a wide range. Molecular Dynamics simulations show that partially adsorbed chains (with "tails", surface attached "trains" and "loops") are not described by the Kratky-Porod wormlike chain model. The crossover of the persistence length from its three-dimensional value $(\ell_p)$ to the enhanced value in two dimensions $(2\ell_p)$ is analyzed, and excluded volume effects are identified for $L \gg \ell_p$. Consequences for the interpretation of experiments are suggested. We verify the prediction that the adsorption threshold scales as $\ell_p^{-1/3}$.

cond-mat.soft↗

arXiv2019

The smectic phase in semiflexible polymer materials: A large scale Molecular Dynamics study

Semiflexible polymers in concentrated lyotropic solution are studied within a bead-spring model by molecular dynamics simulations, focusing on the emergence of a smectic A phase and its properties. We systematically vary the density of the monomeric units for several contour lengths that are taken smaller than the chain persistence length. The difficulties concerning the equilibration of such systems and the choice of appropriate ensemble (constant volume versus constant pressure, where all three linear dimensions of the simulation box can fluctuate independently) are carefully discussed. Using HOOMD-blue on graphics processing units, systems containing more than a million monomeric units are accessible, making it possible to distinguish the order of the phase transitions that occur. While in this model the nematic-smectic transition is continuous, the transition from the smectic phase to a related crystalline structure with true three-dimensional long-range order is clearly of first order. Further, both orientational and positional correlations of monomeric units are studied as well as the order parameters characterizing the nematic, smectic A, and crystalline phases. The analogy between smectic order and one-dimensional harmonic crystals with respect to the behavior of the structure factor is also explored. Finally, the results are put in perspective with pertinent theoretical predictions and possible experiments.

cond-mat.soft↗

arXiv2016

Semiflexible Polymers Under Good Solvent Conditions Interacting With Repulsive Walls

Solutions of semiflexible polymers confined by repulsive planar walls are studied by density functional theory and Molecular Dynamics simulations, to clarify the competition between the chain alignment favored by the wall and the depletion caused by the monomer-wall repulsion. A coarse-grained bead-spring model with bond bending potential is studied, varying both the contour length and the persistence length of the polymers, as well as the monomer concentration in the solution (good solvent conditions are assumed throughout, and solvent molecules are not included explicitly). The profiles of monomer density and pressure tensor components near the wall are studied, and the surface tension of the solution is obtained. While the surface tension slightly decreases with chain length for flexible polymers, it clearly increases with chain length for stiff polymers. Thus, at fixed density and fixed chain length the surface tension also increases with increasing persistence length. Chain ends always are enriched near the wall, but this effect is much larger for stiff polymers than for flexible ones. Also the profiles of the mean square gyration radius components near the wall and the nematic order parameter are studied to clarify the conditions where wall-induced nematic order occurs.

cond-mat.soft↗

arXiv2016

Anomalous fluctuations of nematic order in solutions of semiflexible polymers

The nematic ordering in semiflexible polymers with contour length $L$ exceeding their persistence length $\ell_p$ is described by a confinement of the polymers in a cylinder of radius $r_{eff}$ much larger than the radius $r_ρ$, expected from the respective concentration of the solution. Large scale Molecular Dynamics simulations combined with Density Functional Theory are used to locate the Isotropic-Nematic ($I-N$)-transition and to validate this cylindrical confinement. Anomalous fluctuations, due to chain deflections from neighboring chains in the nematic phase are proposed. Considering deflections as collective excitations in the nematically ordered phase of semiflexible polymers elucidates the origins of shortcomings in the description of the $I-N$ transition by existing theories.

cond-mat.soft↗

arXiv2014

Unconventional ordering behavior of semi-flexible polymers in dense brushes under compression

Using a coarse-grained bead-spring model for semi-flexible macromolecules forming a polymer brush, structure and dynamics of the polymers is investigated, varying chain stiffness and grafting density. The anchoring condition for the grafted chains is chosen such that their first bonds are oriented along the normal to the substrate plane. Compression of such a semi-flexible brush by a planar piston is observed to be a two-stage process: for small compressions the chains contract by "buckling" deformation whereas for larger compression the chains exhibit a collective (almost uniform) bending deformation. Thus, the stiff polymer brush undergoes a 2-nd order phase transition of collective bond reorientation. The pressure, required to keep the stiff brush at a given degree of compression, is thereby significantly smaller than for an otherwise identical brush made of entirely flexible polymer chains! While both the brush height and the chain linear dimension in the z-direction perpendicular to the substrate increase monotonically with increasing chain stiffness, lateral (xy) chain linear dimensions exhibit a maximum at intermediate chain stiffness. Increasing the grafting density leads to a strong decrease of these lateral dimensions, compatible with an exponential decay. Also the recovery kinetics after removal of the compressing piston is studied, and found to follow a power-law / exponential decay with time. A simple mean-field theoretical consideration, accounting for the buckling/bending behavior of semi-flexible polymer brushes under compression, is suggested.

cond-mat.soft↗

arXiv2011

Anomalous Structure and Scaling of Ring Polymer Brushes

A comparative simulation study of polymer brushes formed by grafting at a planar surface either flexible linear polymers (chain length $N_L$) or (non-catenated) ring polymers (chain length $N_R=2 N_L$) is presented. Two distinct off-lattice models are studied, one by Monte Carlo methods, the other by Molecular Dynamics, using a fast implementation on graphics processing units (GPUs). It is shown that the monomer density profiles $ρ(z)$ in the $z$-direction perpendicular to the surface for rings and linear chains are practically identical, $ρ_R(2 N_L, z)=ρ_L(N_L, z)$. The same applies to the pressure, exerted on a piston at hight z, as well. While the gyration radii components of rings and chains in $z$-direction coincide, too, and increase linearly with $N_L$, the transverse components differ, even with respect to their scaling properties: $R_{gxy}^{(L)} \propto N_L^{1/2}$, $R_{gxy}^{(R)} \propto N_L^{0.4}$. These properties are interpreted in terms of the statistical properties known for ring polymers in dense melts.

cond-mat.soft↗

arXiv2011

Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: Molecular Dynamics Simulation

The scission kinetics of bottle-brush molecules in solution and on an adhesive substrate is modeled by means of Molecular Dynamics simulation with Langevin thermostat. Our macromolecules comprise a long flexible polymer backbone with $L$ segments, consisting of breakable bonds, along with two side chains of length $N$, tethered to each segment of the backbone. In agreement with recent experiments and theoretical predictions, we find that bond cleavage is significantly enhanced on a strongly attractive substrate even though the chemical nature of the bonds remains thereby unchanged. We find that the mean bond life time $<τ>$ decreases upon adsorption by more than an order of magnitude even for brush molecules with comparatively short side chains $N=1 ÷4$. The distribution of scission probability along the bonds of the backbone is found to be rather sensitive regarding the interplay between length and grafting density of side chains. The life time $<τ>$ declines with growing contour length $L$ as $<τ>\propto L^{-0.17}$, and with side chain length as $<τ>\propto N^{-0.53}$. The probability distribution of fragment lengths at different times agrees well with experimental observations. The variation of the mean length $L(t)$ of the fragments with elapsed time confirms the notion of the thermal degradation process as a first order reaction.

cond-mat.soft↗

arXiv2010

Spherical Polymer Brushes Under Good Solvent Conditions: Molecular Dynamics Results Compared to Density Functional Theory

A coarse grained model for flexible polymers end-grafted to repulsive spherical nanoparticles is studied for various chain lengths and grafting densities under good solvent conditions, by Molecular Dynamics methods and density functional theory. With increasing chain length the monomer density profile exhibits a crossover to the star polymer limit. The distribution of polymer ends and the linear dimensions of individual polymer chains are obtained, while the inhomogeneous stretching of the chains is characterized by the local persistence lengths. The results on the structure factor of both single chain and full spherical brush as well as the range of applicability of the different theoretical tools are presented. Eventually an outlook on experiments is given.

cond-mat.soft↗

arXiv2008

Scaling exponents of Forced Polymer Translocation through a nano-pore

We investigate several scaling properties of a translocating homopolymer through a thin pore driven by an external field present inside the pore only using Langevin Dynamics (LD) simulation in three dimension (3D). Specifically motivated by several recent theoretical and numerical studies that are apparently at odds with each other, we determine the chain length dependence of the scaling exponents of the average translocation time, the average velocity of the center of mass, $ $, the effective radius of gyration during the translocation process, and the scaling exponent of the translocation coordinate ($s$-coordinate) as a function of the translocation time. We further discuss the possibility that in the case of driven translocation the finite pore size and its geometry could be responsible that the veclocity scaling exponent is less than unity and discuss the dependence of the scaling exponents on the pore geometry for the range of $N$ studied here.

cond-mat.soft↗

arXiv2008

Anomalous diffusion of a tethered membrane: A Monte Carlo investigation

Using a continuum bead-spring Monte Carlo model, we study the anomalous diffusion dynamics of a self-avoiding tethered membrane by means of extensive computer simulations. We focus on the subdiffusive stochastic motion of the membrane's central node in the regime of flat membranes at temperatures above the membrane folding transition. While at times, larger than the characteristic membrane relaxation time $τ_R$, the mean-square displacement of the center of mass of the sheet, $ $, as well as that of its central node, $ $, show the normal Rouse diffusive behavior with a diffusion coefficient $D_N$ scaling as $D_N \propto N^{-1}$ with respect to the number of segments $N$ in the membrane, for short times $t\le τ_R$ we observe a {\em multiscale dynamics} of the central node, $ \propto t^α$, where the anomalous diffusion exponent $α$ changes from $α\approx 0.86$ to $α\approx 0.27$, and then to $α\approx 0.5$, before diffusion turns eventually to normal. By means of simple scaling arguments we show that our main result, $α\approx 0.27$, can be related to particular mechanisms of membrane dynamics which involve different groups of segments in the membrane sheet. A comparative study involving also linear polymers demonstrates that the diffusion coefficient of self-avoiding tethered membranes, containing $N$ segments, is three times smaller than that of linear polymer chains with the same number of segments.

cond-mat.soft↗

arXiv2008

Universal properties of a single polymer chain in slit: Scaling versus MD simulations

We revisit the classical problem of a polymer confined in a slit in both of its static and dynamic aspects. We confirm a number of well known scaling predictions and analyse their range of validity by means of comprehensive Molecular Dynamics simulations using a coarse-grained bead-spring model of a flexible polymer chain. The normal and parallel components of the average end-to-end distance, mean radius of gyration and their distributions, the density profile, the force exerted on the slit walls, and the local bond orientation characteristics are obtained in slits of width $D$ = $4 ÷10$ (in units of the bead radius) and for chain lengths $N=50 ÷300$. We demonstrate that a wide range of static chain properties in normal direction can be described {\em quantitatively} by analytic model - independent expressions in perfect agreement with computer experiment. In particular, the observed profile of confinement-induced bond orientation, is shown to closely match theory predictions. The anisotropy of confinement is found to be manifested most dramatically in the dynamic behavior of the polymer chain. We examine the relation between characteristic times for translational diffusion and lateral relaxation. It is demonstrated that the scaling predictions for lateral and normal relaxation times are in good agreement with our observations. A novel feature is the observed coupling of normal and lateral modes with two vastly different relaxation times. We show that the impact of grafting on lateral relaxation is equivalent to doubling the chain length.

physics.chem-ph↗

arXiv2007

Adsorption of Multi-block and Random Copolymer on a Solid Surface: Critical Behavior and Phase Diagram

The adsorption of a single multi-block $AB$-copolymer on a solid planar substrate is investigated by means of computer simulations and scaling analysis. It is shown that the problem can be mapped onto an effective homopolymer adsorption problem. In particular we discuss how the critical adsorption energy and the fraction of adsorbed monomers depend on the block length $M$ of sticking monomers $A$, and on the total length $N$ of the polymer chains. Also the adsorption of the random copolymers is considered and found to be well described within the framework of the annealed approximation. For a better test of our theoretical prediction, two different Monte Carlo (MC) simulation methods were employed: a) off-lattice dynamic bead-spring model, based on the standard Metropolis algorithm (MA), and b) coarse-grained lattice model using the Pruned-enriched Rosenbluth method (PERM) which enables tests for very long chains. The findings of both methods are fully consistent and in good agreement with theoretical predictions.

cond-mat.soft↗