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Friederike Schmid

Publications and source records attributed to Friederike Schmid.

At least 91 records · Page 5Linked to original sources
arXiv2013

Elastic Properties and Line Tension of Self-Assembled Bilayer Membranes

The elastic properties of a self-assembled bilayer membrane are studied using the self-consistent field theory, applied to a model system composed of flexible amphiphilic chains dissolved in hydrophilic polymeric solvents. Examining the free energy of bilayer membranes with different geometries allows us to calculate their bending modulus, Gaussian modulus, two fourth-order membrane moduli, and the line tension. The dependence of these parameters on the microscopic characteristics of the amphiphilic chain, characterized by the volume fraction of the hydrophilic component, is systematically studied. The theoretical predictions are compared with the results from a simple monolayer model, which approximates a bilayer membrane by two monolayers. Finally the region of validity of the linear elasticity theory is analyzed by examining the higher-order contributions.

physics.bio-ph↗
arXiv2013

Dynamic and Dielectric Response of Charged Colloids in Electrolyte Solutions to External Electric Fields

Computer simulations are used to investigate the response of a charged colloid and its surrounding microion cloud to an external electric field. Both static fields (DC) and alternating fields (AC) are considered. A mesoscopic simulation method is implemented to account in full for hydrodynamic and electrostatic interactions. The response of the system can be characterized by two quantities: the mobility and the polarizability. Due to the interplay of the electrostatic attraction and hydrodynamic drag, the response of the microions close to the colloid surface is different from that of the microions far away from the colloid. Both the mobility and polarizability exhibit a dependency on the frequency of the external fields, which can be attributed to the concentration polarization, the mobility of the microions, and the inertia of microions. The effects of the colloidal charge, the salt concentration, and the frequency of the external fields are investigated systematically.

cond-mat.soft↗
arXiv2013

AC-field-induced Polarization for Uncharged Colloids in Salt Solution: A Dissipative Particle Dynamics Simulation

We study the response of a spherical colloid under alternating electric fields (AC-fields) by mesoscopic simulation method, accounting in full for hydrodynamic and electrostatic interactions. We focus on a special case of uncharged colloids. The main polarization mechanism is the "volume-polarization", where ionic currents are deflected by the core of the uncharged colloid. Specifically, we compute the polarizability of a single colloid and systematically investigate the effect of AC field frequency and salt concentration. The simulation results are compared with predictions from classical Maxwell-Wagner theory and electrokinetic theory.

cond-mat.soft↗
arXiv2013

Monolayer curvature stabilizes nanoscale raft domains in mixed lipid bilayers

According to the lipid raft hypothesis, biological lipid membranes are laterally heterogeneous and filled with nanoscale ordered "raft" domains, which are believed to play an important role for the organization of proteins in membranes. However, the mechanisms stabilizing such small rafts are not clear, and even their existence is sometimes questioned. Here we report the observation of raft-like structures in a coarse-grained molecular model for multicomponent lipid bilayers. On small scales, our membranes demix into a liquid ordered (lo) and a liquid disordered (ld) phase. On large scales, phase separation is suppressed and gives way to a microemulsion-type state that contains nanometer size lo domains in a ld environment. Furthermore, we introduce a mechanism that generates rafts of finite size by a coupling between monolayer curvature and local composition. We show that mismatch between the spontaneous curvatures of monolayers in the lo and ld phase induces elastic interactions, which reduce the line tension between the lo and ld phase and can stabilize raft domains with a characteristic size of the order of a few nanometers. Our findings suggest that rafts in multicomponent bilayers might be closely related to the modulated ripple phase in one-component bilayers.

physics.bio-ph↗
arXiv2012

Dielectric Response of Nanoscopic Spherical Colloids in Alternating Electric Fields: A Dissipative Particle Dynamics Simulation

We study the response of single nanosized spherical colloids in electrolyte solution to an alternating electric field (AC field) by computer simulations. We use a coarse-grained mesoscopic simulation approach that accounts in full for hydrodynamic and electrostatic interactions as well as for thermal fluctuations. The solvent is modeled as a fluid of single Dissipative Particle Dynamics (DPD) beads, and the colloidal particle is modeled as a rigid body made of DPD beads. We compute the mobility and the polarizability of a single colloid and investigate systematically the effect of amplitude and frequency of the AC-fields. Even though the thickness of the Debye layer is not "thin" compared to the radius of the colloid, and the thermal fluctuations are significant, the results are in good agreement with the theoretical prediction of the Maxwell-Wagner-O'Konski theory, especially for uncharged colloids.

cond-mat.soft↗
arXiv2012

Anisotropic flow in striped superhydrophobic channels

We report results of dissipative particle dynamics simulations and develop a semi-analytical theory of an anisotropic flow in a parallel-plate channel with two superhydrophobic striped walls. Our approach is valid for any local slip at the gas sectors and an arbitrary distance between the plates, ranging from a thick to a thin channel. It allows us to optimize area fractions, slip lengths, channel thickness and texture orientation to maximize a transverse flow. Our results may be useful for extracting effective slip tensors from global measurements, such as the permeability of a channel, in experiments or simulations, and may also find applications in passive microfluidic mixing.

physics.flu-dyn↗
arXiv2012

Separation of chiral particles in micro- or nanofluidic channels

We propose a method to separate enantiomers in microfluidic or nanofluidic channels. It requires flow profiles which break chiral symmetry and have regions with high local shear. Such profiles can be generated in channels confined by walls with different hydrodynamic boundary conditions (e.g. slip lengths). Due to a nonlinear hydrodynamic effect, particles with different chirality migrate at different speed and can be separated. The mechanism is demonstrated by computer simulations. We investigate the influence of thermal fluctuations (i.e. the Péclet number) and show that the effect disappears in the linear response regime. The details of the microscopic flow are important and determine which volume forces are necessary to achieve separation.

physics.flu-dyn↗
arXiv2012

Membrane-mediated protein-protein interaction: A Monte Carlo study

We investigate membrane-mediated interactions between transmembrane proteins using coarse-grained models. We compare the effective potential of mean force (PMF) between two proteins, which are always aligned parallel to the z-axis of the simulation box, with those PMFs obtained for proteins with fluctuating orientations. The PMFs are dominated by an oscillatory packing-driven contribution and a smooth attractive hydrophobic mismatch contribution, which vanishes if the hydrophobic length of the protein matches the thickness of the membrane. If protein orientations are allowed to fluctuate, the oscillations are greatly reduced compared to proteins with fixed orientation. Furthermore, the hydrophobic mismatch interaction has a smaller range. Finally, we compare the two-dimensional thickness profiles around two proteins with the predictions from the elastic theory of two coupled monolayers, and find them to be in very good agreement.

physics.bio-ph↗
arXiv2012

Hybrid Lattice Boltzmann / Dynamic Self-Consistent Field Simulations of Microphase Separation and Vesicle Formation in Block Copolymer Systems

We present a hybrid numerical method to introduce hydrodynamics in dynamic self-consistent field (SCF) studies of inhomogeneous polymer systems. It solves a set of coupled dynamical equations: The Navier-Stokes equations for the fluid flow, and SCF-based convection-diffusion equations for the evolution of the local monomer compositions. The Navier-Stokes equaitons are simulated by the lattice Boltzmann method and the dynamic self-consistent equations are solved by a finite difference scheme. Two applications are presented: First, we study microphase separation in symmetric and asymmetric block copolymer melts with various values of shear and bulk viscosities, comparing the results to those obtaiuned with purely diffusive dynamics. Second, we investigate the effect of hydrodynamics on vesicle formation in amphiphilic block copolymer solutions. In agreement with previous studies, hydrodynamic interactions are found to have little effect on the microphase separation at early times, buty they substantially accelerate the process of structure formation at later times. Furthermore, they also contribute to selecting the pathway of vesicle formation, favoring spherical intermediates over aspherical (disklike) ones.

cond-mat.soft↗
arXiv2012

Mesoscopic Simulations of Electroosmotic Flow and Electrophoresis in Nanochannels

We review recent dissipative particle dynamics (DPD) simulations of electrolyte flow in nanochannels. A method is presented by which the slip length $delta_B$ at the channel boundaries can be tuned systematically from negative to infinity by introducing suitably adjusted wall-fluid friction forces. Using this method, we study electroosmotic flow (EOF) in nanochannels for varying surface slip conditions and fluids of different ionic strength. Analytic expressions for the flow profiles are derived from the Stokes equation, which are in good agreement with the numerical results. Finally, we investigate the influence of EOF on the effective mobility of polyelectrolytes in nanochannels. The relevant quantity characterizing the effect of slippage is found to be the dimensionless quantity $κδ_B$, where $1/κ$ is an effective electrostatic screening length at the channel boundaries.

physics.flu-dyn↗
arXiv2012

Are stress-free membranes really 'tensionless'?

In recent years it has been argued that the tension parameter driving the fluctuations of fluid membranes, differs from the imposed lateral stress, the 'frame tension'. In particular, stress-free membranes were predicted to have a residual fluctuation tension. In the present paper, this argument is reconsidered and shown to be inherently inconsistent -- in the sense that a linearized theory, the Monge model, is used to predict a nonlinear effect. Furthermore, numerical simulations of one-dimensional stiff membranes are presented which clearly demonstrate, first, that the internal 'intrinsic' stress in membranes indeed differs from the frame tension as conjectured, but second, that the fluctuations are nevertheless driven by the frame tension. With this assumption, the predictions of the Monge model agree excellently with the simulation data for stiffness and tension values spanning several orders of magnitude.

cond-mat.soft↗
arXiv2011

Collapse transition of flexible homopolymers with adhesive contacts

The presence of ordered structures such as helices in collapsed states of polymer chains is still an open question challenging physics and biology. In this work, we present a potential model for polymer chains with monomers that are not strictly attractive, but that can make adhesive contacts with other monomers. We find that the chain develop helical order during the process of collapsing from an initially stretched conformation. It seems in this case that the adhesive contacts help the polymer chain to stay trapped in the helix state.

cond-mat.soft↗
arXiv2011

A method to compute absolute free energies or enthalpies of fluids

We propose a new method to compute the free energy or enthalpy of fluids or disordered solids by computer simulation . The main idea is to construct a reference system by freezing one representative configuration, and then carry out a thermodynamic integration. We present a strategy and an algorithm which allows to sample the thermodynamic integration path even in the case of liquids, despite the fact that the particles can diffuse freely through the system. The method is described in detail and illustrated with applications to hard sphere fluids and solids with mobile defects.

physics.comp-ph↗
arXiv2010

Mesoscopic simulations of polyelectrolyte electrophoresis in nanochannels

We present the results of mesoscopic dissipative particle dynamics (DPD) simulations of coupled electrohydrodynamic phenomena on the micro- and nanoscale. The effects of electroosmotic flow and slippage combined with polyelectrolyte electrophoresis are investigated in detail, taking full account of hydrodynamic and electrostatic interactions. Our numerical results are in excellent agreement with analytical calculations.

cond-mat.soft↗
arXiv2010

Fluctuations and Elastic Properties of Lipid Membranes in the Tilted Gel State: A Coarse-Grained Monte Carlo Study

We study the stress distribution profiles and the height and thickness fluctuations of lipid membranes in the tilted gel state by Monte Carlo simulations of a generic coarse-grained model for lipid membranes, which reproduces many known properties of DPPC bilayers. The results are related to the corresponding properties of fluid membranes, and to theoretical predictions for crystalline and hexatic membranes. One striking observation is that the spontaneous curvature of the monolayers changes sign from the fluid to the gel phase. In the gel phase, the long-wavelength height fluctuations are suppressed, and the fluctuation spectrum is highly anisotropic. In the direction of tilt, it carries the signature of soft modes that are compatible with the wavelength of the ripple phase, which emerges in the transition region between the fluid and the gel state. In the direction perpendicular to the tilt, the thickness fluctuations are almost entirely suppressed, and the height fluctuations seem to be dominated by an interfacial energy, i.e., by out-of-layer fluctuations, up to length scales of tens of nanometers.

cond-mat.soft↗
arXiv2010

Coarse-Grained Simulations of Membranes under Tension

We investigate the properties of membranes under tension by Monte-Carlo simulations of a generic coarse-grained model for lipid bilayers. We give a comprising overview of the behavior of several membrane characteristics, such as the area per lipid, the monolayer overlap, the nematic order, and pressure profiles. Both the low-temperature regime, where the membranes are in a gel phase, and the high-temperature regime, where they are in the fluid phase, are considered. In the gel state, the membrane is hardly influenced by tension. In the fluid state, high tensions lead to structural changes in the membrane, which result in different compressibility regimes. The ripple state, which is found at tension zero in the transition regime between the fluid and the gel phase, disappears under tension and gives way to an interdigitated phase. We also study the membrane fluctuations in the fluid phase. In the low tension regime the data can be fitted nicely to a suitably extended elastic theory. At higher tensions the elastic fit consistently underestimates the strength of long-wavelength fluctuations. Finally, we investigate the influence of tension on the effective interaction between simple transmembrane inclusions and show that tension can be used to tune the hydrophobic mismatch interaction between membrane proteins.

physics.bio-ph↗
arXiv2010

Theory and Simulation of Multiphase Polymer Systems

The theory of multiphase polymer systems has a venerable tradition. The 'classical' theory of polymer demixing, the Flory-Huggins theory, was developed already in the forties of the last century. It is still the starting point for most current approaches -- be they improved theories for polymer (im)miscibility that take into account the microscopic structure of blends more accurately, or sophisticated field theories that allow to study inhomogeneous multicomponent systems of polymers with arbitrary architectures in arbitrary geometries. In contrast, simulations of multiphase polymer systems are relatively young. They are still limited by the fact that one must simulate a large number of large molecules in order to obtain meaningful results. Both powerful computers and smart modeling and simulation approaches are necessary to overcome this problem. This article gives an overview over the state-of-the art in both areas, theory and simulation. While the theory has reached a fairly mature stage by now, and many aspects of it are covered in textbooks on polymer physics, the information on simulations is much more scattered. This is why some effort has been invested into putting together a representative list of references in this area (up to the year of 2008) -- which is of course still far from complete.

cond-mat.soft↗