Novel structure formation of a phase separating colloidal fluid in a ratchet potential

Abstract

PACS 64.75.Xc -Phase separation and segregation in colloidal systems PACS 47.54.-r -Pattern selection; pattern formation PACS 64.70.Nd -Structural transitions in nanoscale materials Abstract -Based on Dynamical Density Functional Theory (DDFT) we investigate a binary mixture of interacting Brownian particles driven over a substrate via a one-dimensional ratchet potential. The particles are modeled as soft spheres where one component carries a classical Heisenberg spin. In the absence of a substrate field, the system undergoes a first-order fluidfluid demixing transition driven by the spin-spin interaction. We demonstrate that the interplay between the intrinsic spinodal decomposition and time-dependent external forces leads to a novel dynamical instability where stripes against the symmetry of the external potential form. This structural transition is observed for a broad range of parameters related to the ratchet potential. Moreover, we find intriguing effects for the particle transport.Introduction. -Understanding the dynamics of particles in complex geometry is an ubiquitary problem throughout non-equilibrium statistical physics with applications in diverse fields such as biology, condensed matter and nanotechnology [1,2]. Paradigm examples are colloidal particles in periodic optical (or otherwise modulated) potentials [3][4][5], which display a variety of fascinating transport phenomena including giant diffusion [6], subdiffusive motion [7], and ratchet effects, i.e., fluctuatinginduced transport in the absence of a biasing deterministic force [8]. Indeed, ratchet-driven transport of Brownian (overdamped) particles has been studied in a variety of optical [9,10], magnetic [11][12][13][14][15][16], and biological systems [17][18][19]. The advantage of studying colloids, which are typically of the size of nano-to micrometer, is that many of these effects can be monitored by real-space experiments (see, e.g.,). In the present letter we study the impact of ratchet potentials on the collective behavior, specifically the phase separation dynamics, of a colloidal suspension. As a model system we consider systems involving magnetic colloids subject to magnetic ratchet potentials. Indeed, recent experimental and theoretical research has shown that magnetic colloidal systems are ideally suited to study transport in complex geometries. Static, magnetic periodic potentials can be created, e.g., by using ferrite garnet films

Lane Formation Dynamics of Oppositely Self-Driven Binary Particles: Effects of Density and Finite System Size

“…A similar lane-formation phenomenon has also been demonstrated in simulations of binary oppositely charged particles [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] as well as experiments [32][33][34][35]. The phase diagram of the lane formation has been examined for various combinations of the external force and the density under the * kk@cheng.es.osaka-u.ac.jp periodic boundary condition [17][18][19][20][21]31] or with confined geometry [27,29].…”

Collective dynamics of dipolar and multipolar colloids: From passive to active systems

“…species. As shown in [100], anisotropic dipolar interactions induce several novel features within the (otherwise well-studied) laning transition. In particular, laning occurs only in a window of interaction strengths, Fig.…”

Lane formation in a driven attractive fluid

“…Here, we use the DDFT as a starting point for the stability analysis of the 'equilibrium' system defined by the effective interaction potential U eff (r ij ). Due to the attractive terms in U eff (r ij ) one indeed expects instabilities related to fluctuations of the total density (e.g., condensation) or concentration (e.g., demixing) [37].…”