arXiv subjects

S. Dietrich

Publications and source records attributed to S. Dietrich.

At least 91 records · Page 5Linked to original sources

arXiv2012

Precursor films in wetting phenomena

The spontaneous spreading of non-volatile liquid droplets on solid substrates poses a classic problem in the context of wetting phenomena. It is well known that the spreading of a macroscopic droplet is in many cases accompanied by a thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter with a much higher speed. Such films have been usually associated with liquid-on-solid systems, but in the last decade similar films have been reported to occur in solid-on-solid systems. While the situations in which the thickness of such films is of mesoscopic size are rather well understood, an intriguing and yet to be fully understood aspect is the spreading of microscopic, i.e., molecularly thin films. Here we review the available experimental observations of such films in various liquid-on-solid and solid-on-solid systems, as well as the corresponding theoretical models and studies aimed at understanding their formation and spreading dynamics. Recent developments and perspectives for future research are discussed.

cond-mat.soft↗

arXiv2012

Micro- and nanoscale fluid flow on chemical channels

We study the time evolution and driven motion of thin liquid films lying on top of chemical patterns on a substrate. Lattice-Boltzmann and molecular dynamics methods are used for simulations of the flow of microscopic and nanoscopic films, respectively. Minimization of fluid surface area is used to examine the corresponding equilibrium free energy landscapes. The focus is on motion across patterns containing diverging and converging flow junctions, with an eye towards applications to lab-on-a-chip devices. Both open liquid-vapor systems driven by body forces and confined liquid-liquid systems driven by boundary motion are considered. As in earlier studies of flow on a linear chemical channel, we observe continuous motion of a connected liquid film across repeated copies of the pattern, despite the appearance of pearling instabilities of the interface. Provided that the strength of the driving force and the volume of liquid are not too large, the liquid is confined to the chemical channels and its motion can be directed by small variations in the geometry of the pattern.

cond-mat.soft↗

arXiv2012

Complete wetting of elastically responsive substrates

We analyze theoretically complete wetting of a substrate supporting an array of parallel, vertical plates which can tilt elastically. The adsorbed liquid tilts the plates, inducing clustering, and thus modifies the substrate geometry. In turn, this change in geometry alters the wetting properties of the substrate and, consequently, the adsorption of liquid. This geometry-wetting feedback loop leads to stepped adsorption isotherms with each step corresponding to an abrupt change in the substrate geometry. We discuss how this can be used for constructing substrates with tunable wetting and adsorption properties.

cond-mat.soft↗

arXiv2012

Phase behavior of colloidal suspensions with critical solvents in terms of effective interactions

We study the phase behavior of colloidal suspensions the solvents of which are considered to be binary liquid mixtures undergoing phase segregation. We focus on the thermodynamic region close to the critical point of the accompanying miscibility gap. There, due to the colloidal particles acting as cavities in the critical medium, the spatial confinements of the critical fluctuations of the corresponding order parameter result in the effective, so-called critical Casimir forces between the colloids. Employing an approach in terms of effective, one-component colloidal systems, we explore the possibility of phase coexistence between two phases of colloidal suspensions, one being rich and the other being poor in colloidal particles. The reliability of this effective approach is discussed.

cond-mat.soft↗

arXiv2012

Structure and aggregation of colloids immersed in critical solvents

We consider an ensemble of spherical colloidal particles immersed in a near-critical solvent such as a binary liquid mixture close to its critical demixing point. The emerging long-ranged fluctuations of the corresponding order parameter of the solvent drive the divergence of the correlation length. Spatial confinements of these critical fluctuations by colloidal solute particles, acting as cavities in the fluctuating medium, restrict and modify the fluctuation spectrum in a way which depends on their relative configuration. This results in effective, so-called critical Casimir forces (CCFs) acting on the confining surfaces. Using the available knowledge about CCFs we study the structure and stability of such colloidal suspensions by employing an approach in terms of effective, one-component colloidal systems. Applying the approximation of pairwise additive CCFs we calculate the radial distribution function of the colloids, which is experimentally accessible. We analyze colloidal aggregation due to CCFs and thus allude to previous experimental studies which are still under debate

cond-mat.soft↗

arXiv2011

Collective dynamics of colloids at fluid interfaces

The evolution of an initially prepared distribution of micron sized colloidal particles, trapped at a fluid interface and under the action of their mutual capillary attraction, is analyzed by using Brownian dynamics simulations. At a separation λ given by the capillary length of typically 1 mm, the distance dependence of this attraction exhibits a crossover from a logarithmic decay, formally analogous to two-dimensional gravity, to an exponential decay. We discuss in detail the adaption of a particle-mesh algorithm, as used in cosmological simulations to study structure formation due to gravitational collapse, to the present colloidal problem. These simulations confirm the predictions, as far as available, of a mean-field theory developed previously for this problem. The evolution is monitored by quantitative characteristics which are particularly sensitive to the formation of highly inhomogeneous structures. Upon increasing λ the dynamics show a smooth transition from the spinodal decomposition expected for a simple fluid with short-ranged attraction to the self-gravitational collapse scenario.

cond-mat.soft↗

arXiv2011

Shock waves in capillary collapse of colloids: a model system for two--dimensional screened Newtonian gravity

Using Brownian dynamics simulations, density functional theory, and analytical perturbation theory we study the collapse of a patch of interfacially trapped, micrometer-sized colloidal particles, driven by long-ranged capillary attraction. This attraction {is formally analogous} to two--dimensional (2D) screened Newtonian gravity with the capillary length \hatλ as the screening length. Whereas the limit \hatλ \to \infty corresponds to the global collapse of a self--gravitating fluid, for finite \hatλ we predict theoretically and observe in simulations a ringlike density peak at the outer rim of a disclike patch, moving as an inbound shock wave. Possible experimental realizations are discussed.

cond-mat.soft↗

arXiv2011

Critical Casimir forces for Ising films with variable boundary fields

Monte Carlo simulations based on an integration scheme for free energy differences is used to compute critical Casimir forces for three-dimensional Ising films with various boundary fields. We study the scaling behavior of the critical Casimir force, including the scaling variable related to the boundary fields. Finite size corrections to scaling are taken into account. We pay special attention to that range of surface field strengths within which the force changes from repulsive to attractive upon increasing the temperature. Our data are compared with other results available in the literature.

cond-mat.stat-mech↗

arXiv2011

Pulling and Pushing a Cargo With a Catalytically Active Carrier

Catalytically active particles suspended in a liquid can move due to self-phoresis by generating solute gradients via chemical reactions of the solvent occurring at parts of their surface. Such particles can be used as carriers at the micro-scale. As a simple model for a carrier-cargo system we consider a catalytically active particle connected by a thin rigid rod to a catalytically inert cargo particle. We show that the velocity of the composite strongly depends on the relative orientation of the carrier-cargo link. Accordingly, there is an optimal configuration for the linkage. The subtlety of such carriers is underscored by the observation that a spherical particle completely covered by catalyst, which is motionless when isolated, acts as a carrier once attached to a cargo.

cond-mat.soft↗

arXiv2011

Magnetization of multicomponent ferrofluids

The solution of the mean spherical approximation (MSA) integral equation for isotropic multicomponent dipolar hard sphere fluids without external fields is used to construct a density functional theory (DFT), which includes external fields, in order to obtain an analytical expression for the external field dependence of the magnetization of ferrofluidic mixtures. This DFT is based on a second-order Taylor series expansion of the free energy density functional of the anisotropic system around the corresponding isotropic MSA reference system. The ensuing results for the magnetic properties are in quantitative agreement with our canonical ensemble Monte Carlo simulation data presented here.

cond-mat.stat-mech↗

arXiv2011

Electrostatic interactions in critical solvents

The subtle interplay between critical phenomena and electrostatics is investigated by considering the effective force acting on two parallel walls confining a near-critical binary liquid mixture with added salt. The ion-solvent coupling can turn a non-critical repulsive electrostatic force into an attractive one upon approaching the critical point. However, the effective force is eventually dominated by the critical Casimir effect, the universal properties of which are not altered by the presence of salt. This observation allows a consistent interpretation of recent experimental data.

cond-mat.soft↗

arXiv2011

Capillary interactions in Pickering emulsions

The effective capillary interaction potentials for small colloidal particles trapped at the surface of liquid droplets are calculated analytically. Pair potentials between capillary monopoles and dipoles, corresponding to particles floating on a droplet with a fixed center of mass and subjected to external forces and torques, respectively, exhibit a repulsion at large angular separations and an attraction at smaller separations, with the latter resembling the typical behavior for flat interfaces. This change of character is not observed for quadrupoles, corresponding to free particles on a mechanically isolated droplet. The analytical results for quadrupoles are compared with the numerical minimization of the surface free energy of the droplet in the presence of ellipsoidal particles.

cond-mat.soft↗

arXiv2010

Effective interactions and equilibrium configurations of colloidal particles on a sessile droplet

We study the free energy landscapes of a pair of submicron spherical particles floating at the surface of a sessile droplet. The particles are subjected to radial external forces resulting in a deformation of the droplet shape relative to the reference shape of a spherical cap. This deformation leads to tangential forces on the particles. For small deformations and for the contact angle $θ_0$ at the substrate being equal to $π/2$, the corresponding linearized Young-Laplace equation is solved analytically. The solution is constructed by employing the method of images from electrostatics, where each of the particles plays the role of a capillary monopole and the substrate is replaced by a virtual drop with image charges and by imposing the conditions of fixed droplet volume and vanishing total force on the droplet. The substrate boundary conditions determine the signs of the image capillary charges and therefore also the strength of the tangential forces on the particles. In the cases of an arbitrary contact angle $θ_0$ these forces are calculated numerically by employing a finite element method to find the equilibrium shape of the droplet for those configurations in which the particles are close to the local free energy minima.

cond-mat.soft↗

arXiv2010

Critical Casimir forces steered by patterned substrates

Among the various kinds of effective forces in soft matter, the spatial range and the direction of the so-called critical Casimir force - which is generated by the enhanced thermal fluctuations close to a continuous phase transition - can be controlled and reversibly modified to an uncommonly large extent. In particular, minute temperature changes of the fluid solvent, which provides the near-critical thermal fluctuations, lead to a significant change of the range and strength of the effective interaction among the solute particles. This feature allows one to control, e.g., the aggregation of colloidal dispersions or the spatial distribution of colloids in the presence of chemically or topographically patterned substrates. The spatial direction of the effective force acting on a solute particle depends only on the surface properties of the immersed particles and can be spatially modulated by suitably patterned surfaces. These critical Casimir forces are largely independent of the specific materials properties of both the solvent and the confining surfaces. This characteristic universality of critical phenomena allows systematic and quantitative theoretical studies of the critical Casimir forces in terms of suitable representative and simplified models. Here we highlight recent theoretical and experimental advances concerning critical Casimir forces with a particular emphasis on the numerous possibilities of controlling these forces by substrate patterns.

cond-mat.soft↗

arXiv2010

Critical Casimir forces involving a chemically structured substrate

Motivated by recent experiments with confined binary liquid mixtures near their continous demixing phase transition we study the critical behavior of a system, which belongs to the Ising universality class, for the film geometry with one planar wall chemically structured such that there is a laterally alternating adsorption preference for the species of the binary liquid mixture. By means of Monte Carlo simulations and finite-size scaling analysis we determine the critical Casimir force and the corresponding universal scaling function.

cond-mat.soft↗

arXiv2010

Free energy of colloidal particles at the surface of sessile drops

The influence of finite system size on the free energy of a spherical particle floating at the surface of a sessile droplet is studied both analytically and numerically. In the special case that the contact angle at the substrate equals $π/2$ a capillary analogue of the method of images is applied in order to calculate small deformations of the droplet shape if an external force is applied to the particle. The type of boundary conditions for the droplet shape at the substrate determines the sign of the capillary monopole associated with the image particle. Therefore, the free energy of the particle, which is proportional to the interaction energy of the original particle with its image, can be of either sign, too. The analytic solutions, given by the Green's function of the capillary equation, are constructed such that the condition of the forces acting on the droplet being balanced and of the volume constraint are fulfilled. Besides the known phenomena of attraction of a particle to a free contact line and repulsion from a pinned one, we observe a local free energy minimum for the particle being located at the drop apex or at an intermediate angle, respectively. This peculiarity can be traced back to a non-monotonic behavior of the Green's function, which reflects the interplay between the deformations of the droplet shape and the volume constraint.

cond-mat.soft↗

arXiv2010

Colloidal aggregation and critical Casimir forces

A recent Letter [Phys. Rev. Lett. 103, 156101 (2009)] reports the experimental observation of aggregation of colloidal particles dispersed in a liquid mixture of heavy water and 3-methylpyridine. The experimental data are interpreted in terms of a model which accounts solely for the competing effects of the interparticle electrostatic repulsion and of the attractive critical Casimir force. Here we show, however, that the reported aggregation actually occurs within ranges of values of the correlation length and of the Debye screening length ruled out by the proposed model and that a significant part of the experimental data presented in the Letter cannot be consistently interpreted in terms of such a model.

cond-mat.soft↗

arXiv2010

Dynamics of colloidal particles with capillary interactions

We investigate the dynamics of colloids at a fluid interface driven by attractive capillary interactions. At submillimeter length scales, the capillary attraction is formally analogous to two-dimensional gravity. In particular it is a non-integrable interaction and it can be actually relevant for collective phenomena in spite of its weakness at the level of the pair potential. We introduce a mean-field model for the dynamical evolution of the particle number density at the interface. For generic values of the physical parameters the homogeneous distribution is found to be unstable against large-scale clustering driven by the capillary attraction. We also show that for the instability to be observable, the appropriate values for the relevant parameters (colloid radius, surface charge, external electric field, etc.) are experimentally well accessible. Our analysis contributes to current studies of the structure and dynamics of systems governed by long-ranged interactions and points towards their experimental realizations via colloidal suspensions.

cond-mat.soft↗