We study hydrodynamic slowing-down of a particle moving in a temperature gradient perpendicular to a wall. At distances much smaller than the particle radius, $h\ll a$, lubrication approximation leads to the reduced velocity $u/u_{0}=3\frac{h}{a}\ln \frac{a}{h}$, with respect to the bulk value $u_{0}$. With Brenner's result for confined diffusion, we find that the trapping efficiency, or effective Soret coefficient, increases logarithmically as the particle gets very close to the wall. This provides a quantitative explanation for the recently observed enhancement of thermophoretic trapping at short distances. Our discussion of parallel and perpendicular thermophoresis in a capillary, reveals a very a good agreement with five recent experiments on charged polystyrene particles.
We use a hydrodynamic reciprocal approach to phoretic motion to derive general expressions for the electrophoretic and thermophoretic mobility of weakly charged colloids in aqueous electrolyte solutions. Our approach shows that phoretic motion can be understood in terms of the interfacial transport of thermodynamic excess quantities that arises when a colloid is kept stationary inside a bulk fluid flow. The obtained expressions for the mobilities are extensions of previously known results as they can account for different hydrodynamic boundary conditions at the colloidal surface, irrespective …
Colloidal migration in temperature gradient is referred to as thermophoresis. In contrast to particles with spherical shape, we show that elongated colloids may have a thermophoretic response that varies with the colloid orientation. Remarkably, this can translate into a non-vanishing thermophoretic force in the direction perpendicular to the temperature gradient. Oppositely to the friction force, the thermophoretic force of a rod oriented with the temperature gradient can be larger or smaller than when oriented perpendicular to it. The precise anisotropic thermophoretic behavior clearly depen…
We study here how phase transitions are induced in colloidal suspensions by a point-like heat source, an optically trapped metal oxide particle absorbing light. We find that thermophoresis increases the number density of colloids around the oxide particle, leading to the appearance of solid clusters. Our analysis based on thermophoresis reveals that the solid-fluid interface position is purely determined by the relationship of the particle concentration profile in the fluid state with the volume fraction of the phase transition, and no other effect of thermodynamics is seen in the cluster size…
It has been recently shown that a colloidal monolayer, e.g., formed at a fluid interface or by means of a suitable confining potential, exhibits anomalous collective diffusion. This is a consequence of the hydrodynamic interactions mediated by the three-dimensional (3D) ambient fluid when the particles are confined to reside on a two-dimensional (2D) manifold. We study theoretically and with numerical simulations the crossover from normal to anomalous diffusion as the particles are, in real systems, confined by a 3D external potential and thus have the possibility to fluctuate out of the 2D ma…
From studies via Molecular Dynamics simulations, we report results on structure and dynamics in mixtures of active colloids and passive polymers that are confined inside a spherical container with a repulsive boundary. Such systems mimic the presence of bacteria in the background of bio-polymers. All interactions in the fully passive case are chosen in such a way that in equilibrium coexistence between colloid-rich and polymer-rich phases occurs. For most part of the studies the chosen compositions give rise to Janus structure; one side of the sphere is occupied by the colloids and the rest by…
We investigate the tagged-particle motion in a strongly interacting quasi-confined liquid using periodic boundary conditions along the confining direction. Within a mode-coupling theory of the glass transition (MCT) we calculate the self-nonergodicity parameters and the self-intermediate scattering function and compare them with event-driven molecular dynamics simulations. We observe non-monotonic behavior for the in-plane mean-square displacement and further correlation functions which refer to higher mode indices encoding information about the perpendicular motion. The in-plane velocity-auto…
A binary mixture of super-paramagnetic colloidal particles is confined between glass plates such that the large particles become fixed and provide a two-dimensional disordered matrix for the still mobile small particles, which form a fluid. By varying fluid and matrix area fractions and tuning the interactions between the super-paramagnetic particles via an external magnetic field, different regions of the state diagram are explored. The mobile particles exhibit delocalized dynamics at small matrix area fractions and localised motion at high matrix area fractions, and the localization transiti…
Recent theories predict that when a supercooled liquid approaches the glass transition, particle clusters with a special "amorphous order" nucleate within the liquid, which lead to static correlations dictating the dramatic slowdown of liquid relaxation. The prediction, however, has yet to be verified in 3D experiments. Here, we design a colloidal system, where particles are confined inside spherical cavities with an amorphous layer of particles pinned at the boundary. Using this novel system, we capture the amorphous-order particle clusters and demonstrate the development of a static correlat…
We calculate the configurational entropy of colloidal particles in a confined geometry interacting as hard disks using Monte Carlo integration method. In particular, we consider systems with three kinds of boundary conditions: hard, periodic and spherical. For small to moderate packing fraction $ϕ$ values, we find the entropies per particle for systems with the periodic and spherical boundary conditions tend to reach a same value with the increase of the particle number $N$, while that for the system with the hard boundary conditions still has obvious differences compared to them within the st…
Based on Brownian dynamics simulations, we investigate the thermodynamic signatures of non-equilibrium steady states in a confined colloidal suspensions under shear flow. Specifically, we consider a thin film consisting of charged particles in narrow slit-pore confinement, forming two layers with quadratic in-plane structure in equilibrium. This many-body system displays three distinct steady states, characterized by unique dynamical and rheological response to the applied shear flow. Calculating the work and heat, we find that both quantities indicate the different states by their mean and by…
Patchy colloidal model with three and four equivalent patches, confined in the attractive random porous media, undergo re-entrant gas-liquid phase separation with the possibility for the liquid phase density to approach zero. This unusual behavior is caused by an interplay between strong fluid-fluid bonding interactions and weak fluid-matrix attractions. At high temperature the shape of the phase diagram is determined by the attractive interaction between the fluid particles; weak Yukawa attraction between fluid and obstacles only slightly enhances the fluid-fluid bonding. At low enough temper…
Exploring the role of hydrodynamic interactions in spherically-confined drying colloidal suspensions
Auburn University（奥本大学）；Leibniz-Institut für Polymerforschung Dresden e.V.（德累斯顿莱布尼茨聚合物研究所）；Technische Universität Dresden（德累斯顿工业大学）；Johannes Gutenberg University Mainz（美因茨约翰内斯·古腾堡大学）
Effects of rigid or adaptive confinement on colloidal self-assembly. Fixed vs. fluctuating number of confined particles
在[J. Pakkalski et al. J. Chem. Phys. 142，014903（2015）]中的大正则方程组（GCE）中精确求解的一维晶格模型中，研究了在固定数量的受限粒子的情况下，受限对胶体自组装的影响。该模型考虑了由短程吸引加上长程排斥定义的对相互作用。我们考虑与自组装成团簇相对应的热力学状态。固定和自适应边界进行了研究。对于固定的边界，有一些特殊的状态，在这些状态中，对于相等的平均密度，GCE中的团簇数量大于正则包围圈中的团簇数量。当系统尺寸随颗粒数的变化而变化时，压力对密度的依赖关系与当颗粒数随系统尺寸的变化而变化时有不同的形式。在前一种情况下，压力对系统尺寸有非单调的依赖关系。膨胀系统压力的异常增加伴随着大量小团簇的形成。在弹性约束面的情况下，我们观察到一个双稳态，即两个显着不同的系统大小发生几乎相同的概率。在封闭系统中的双稳态的机制是不同的情况下的渗透壁，其中两个平衡系统的大小对应于不同数量的颗粒。
We perform computational investigations of electrolyte-mediated interactions of charged colloidal particles confined within nanochannels. We investigate the role of discrete ion effects, valence, and electrolyte strength on colloid-wall interactions. We find for some of the multivalent charge regimes that the like-charged colloids and walls can have attractive interactions. We study in detail these interactions and the free energy profile for the colloid-wall separation. We find there are energy barriers and energy minima giving preferred colloid locations in the channel near the center and at…
Interface localization-delocalization transitions (ILDT) occur in two-phase fluids confined in a slit with competing preferences of the walls for the two fluid phases. At low temperatures the interface between the two phases is localized at one of the walls. Upon increasing temperature it unbinds. Although intensively studied theoretically and computationally, such transitions have not yet been observed experimentally due to severe challenges in resolving fine details of the fluid structure. Here, using mean field theory and Monte Carlo simulations of the Ising model, we propose to detect thes…
We report on the linear viscoelastic properties of mixtures comprising multiarm star (as model soft colloids) and long linear chain homopolymers in a good solvent. In contrast to earlier works, we investigated symmetric mixtures (with a size ratio of 1) and showed that the polymeric and colloidal responses can be decoupled. The adopted experimental protocol involved probing the linear chain dynamics in different star environments. To this end, we studied mixtures with different star mass fraction, which was kept constant while linear chains were added and their entanglement plateau modulus ($G…
Using Brownian dynamics (BD) simulations and an analytical approach we investigate the shear-induced, nonequilibrium dynamics of dense colloidal suspensions confined to a narrow slit-pore. Focusing on situations where the colloids arrange in well-defined layers with solidlike in-plane structure, the confined films display complex, nonlinear behavior such as collective depinning and local transport via density excitations. These phenomena are reminiscent of colloidal monolayers driven over a periodic substrate potential. In order to deepen this connection, we present an effective model which ma…
We explore the microstructure and phase behavior of confined soft colloids which can actively switch their interactions at a predefined kinetic rate. For this, we employ a reaction-diffusion approach based on a reactive dynamical density-functional theory (R-DDFT) and study the effect of a binary (two-state) switching of the size of colloids interacting with a Gaussian pair potential. The rate of switching interpolates between a near-equilibrium binary Gaussian mixture at low rates and an effective monodisperse liquid for large rates, hence strongly affecting the one-body density profiles, ads…
We present numerical results for the tagged-particle dynamics by solving the mode-coupling theory in confined geometry for colloidal liquids (cMCT). We show that neither the microscopic dynamics nor the type of intermediate scattering function qualitatively changes the asymptotic dynamics in vicinity of the glass transition. In particular, we find similar characteristics of confinement in the low-frequency susceptibility spectrum which we interpret as footprints of parallel relaxation. We derive predictions for the localization length and the scaling of the diffusion coefficient in the superco…
We introduce a Rigid-Body Fluctuating Immersed Boundary (RB-FIB) method to perform large-scale Brownian dynamics simulations of suspensions of rigid particles in fully confined domains, without any need to explicitly construct Green's functions or mobility operators. In the RB-FIB approach, discretized fluctuating Stokes equations are solved with prescribed boundary conditions in conjunction with a rigid-body immersed boundary method to discretize arbitrarily-shaped colloidal particles with no-slip or active-slip prescribed on their surface. We design a specialized Split--Euler--Maruyama tempo…
In many natural and artificial devices diffusive transport takes place in confined geometries with corrugated boundaries. Such boundaries cause both entropic and hydrodynamic effects, which have been studied only for the case of spherical particles. Here we experimentally investigate diffusion of particles of elongated shape confined into a corrugated quasi-two-dimensional channel. Elongated shape causes complex excluded-volume interactions between particle and channel walls which reduce the accessible configuration space and lead to novel entropic free energy effects. The extra rotational deg…
