We report on the hydrodynamic correlations between colloids immersed in a low Reynolds number fluid. We consider colloidal arrays composed of three particles; each colloid is trapped in a single harmonic potential and interacts with the other colloids only via hydrodynamic forces. We focus on the role of a third body in the two-body correlation functions. We give special attention to a collinear configuration of particles, although the salient features of an equilateral triangle configuration are outlined. The correlation functions are computed both by means of Brownian dynamics simulations, and by solving analytically and numerically the Langevin equation under the assumption of constant diffusion tensor; this approximation is validated through computer simulations. We explicitly show that the presence of a third body affects the auto- and cross-correlation functions and that their behaviour, in some specific conditions, can be different from that commonly seen in a two-particle system. In particular, we have found that the auto-correlation functions show a slower decay, while the cross-correlation ones exhibit a temporal shift and a weaker amplitude. Moreover, an unexpected behaviour related to a positive correlation and associated with the appearance of new dynamical modes is observed in the case of the collinear array of three particles. This interesting effect might be used to tune the degree of hydrodynamic correlation in few-body colloidal systems.
Interactions between membranes and molecules are important for many biological processes, e.g., transport of molecules across cell membranes. However, the detailed physical description of the membrane-biomolecule system remains a challenge and simplified schemes allow capturing its main intrinsic features. In this work, by means of Monte Carlo computer simulations, we systematically study the distribution of uncharged spherical molecules in contact with a flexible surface. Our results show that the distribution for finite size particles has the same simple functional form as the one obtained for point-like particles and depends only on the ratio of the lateral correlation length of the membrane and the radius of the molecules.
Colloids are present in a large variety of biological, chemical and physical systems. In the last few years, they have been used as model systems which allow understanding fundamental processes in atomic systems or elucidating problems in soft condensed matter The success of colloids to be used as well-controlled model systems resides in the fact that the relevant interactions between colloids are easily and independently tuneable and the colloid position is accessible by means of optical techniques, thus allowing a direct comparison with simulations and theoretical calculations. In this contribution, we briefly show the versatility of colloids as model systems to, on one hand, understand the effective interactions that emerge in soft matter physics when unobservable components of the system are integrated out or contracted of the description and, on the other hand, to quantify the effects of soft and periodic external fields on the structural and dynamics properties of many-body systems. EVTRODUCTIONColloids are mesoscopic objects with a size between IQnm to a few jim which possess, either natural or artificial, different shapes (spherical and non-spherical) and typically are dispersed in an aqueous environment [1]. Besides their industrial, chemical and biological relevance, colloids permit to highlight the basic principles and mechanisms in many-body systems, i.e., complex fluids, with competing attractive and repulsive interactions. By adjusting the type of interaction between coUoids, a rich variety of ergodic and non-ergodic transitions can be observed. Moreover, colloidal dispersions under external fields (or substrates) have illustrated the importance of the ordering and dynamics of atomic systems on surfaces such as atomic monolayers [2]. From experimental point of view, particle-particle and particle-substrate can be both realized in a large variety and modified continuously [3]. These extraordinary experimental features together with the striking advantage that inherent properties can be studied simultaneously with complementary methods, such as theory and computer simulations, make to coUoids to be considered as exceUent model systems for soft condensed matterIn this paper, we briefly discuss the effective interactions that emerge in soft matter physics when unobservable components of the system are integrated out or contracted of the description. Also, we study the effects of soft and periodic external fields on the stmcture and dynamics of many-body systems. In particular, in section 11 we investigate the depletion forces between two large hard-colloids immersed in a bath of small hard-colloids [4]; we basically emphasize the role of bridge functions on the depletion CP979,
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