The ability to generate regular spatial arrangements of particles on different length scales is one of the central issues of the "bottom-up" approach in nanotechnology. Current techniques rely on single atom or molecule manipulation by the STM, colloidal particle manipulation by laser or optoelectronic tweezers, microfluidics, optofluidics, micromanipulation and classical lithography. Of particular interest is self-assembly, where the pre-determined spatial arrangements of particles, such as 3D photonic crystals, could be realized spontaneously. Dispersions of particles in liquid crystals show several novel classes of anisotropic forces between inclusions, which result in an amazing diversity of self-assembled patterns, such as linear chains and 2D photonic crystals of microspheres. The forces between the particles in nematic colloids are extremely strong and long-range, resulting in several thousand times stronger binding compared to the binding in water based colloids. The mechanisms of self-assembly in nematic colloids are discussed, showing this is a novel paradigm in colloidal science, which can lead to new approaches in colloidal self-assembly for photonic devices.

Due to their small dimensions, microfluidic devices operate in the low Reynolds number regime. In this case, the hydrodynamics is governed by the viscosity rather than inertia and special elements have to be introduced into the system for mixing and pumping of fluids. Here we report on the realization of an effective pumping device that mimics a ciliated surface and imitates its motion to generate fluid flow. The artificial biomimetic cilia are constructed as long chains of spherical superparamagnetic particles, which selfassemble in an external magnetic field. Magnetic field is also used to actuate the cilia in a simple nonreciprocal manner, resulting in a fluid flow. We prove the concept by measuring the velocity of a cilia-pumped fluid as a function of height above the ciliated surface and investigate the influence of the beating asymmetry on the pumping performance. A numerical simulation was carried out that successfully reproduced the experimentally obtained data.biomimetics | microfluidics | colloids | low Reynold's number | hydrodynamics E fficient pumping and mixing of fluids in microscopic channels is paramount in microfluidic applications (1, 2). Small characteristic dimensions of such devices result in very low Reynolds numbers and one encounters hydrodynamics that is conceptually different from the turbulent macroscopic world. As stated by Purcell's "scallop theorem," nonreciprocal motion is required for generation of fluid flow or directed swimming (3). This is clearly manifested in biological systems, for instance in bending waves of sperm tails or in corkscrew motion of bacterial flagella. Another example are cilia, flexible protrusions on the surface of many eukaryotic cells with a typical length of several micrometers. In humans, ciliated surfaces are found, for example, in the respiratory tract where they sweep mucus, or in the Fallopian tubes where they move an ovum to the uterus. The motion of the fluid above a ciliated surface is generated by periodic beating of cilia. Experimental observations have shown that the beating pattern of an individual cilium is asymmetric and composed of two phases: the effective stroke, during which the outstretched cilium propels the fluid like an oar, followed by the recovery stroke, when the bent cilium returns to the initial position sweeping along the surface in a way that produces as little backward flow as possible (4). Although each cilium can beat independently, cilia densely covering a surface synchronize their cycles and form metachronal waves, thus increasing their fluid pumping efficiency (5). It is believed that the metachronal waves occur as a result of hydrodynamical interactions between the cilia (6, 7).The efficiency of the ciliary pumping mechanism leads to the idea of using the same principle for designing artificial cilia that act as microscale pumps and mixers. An important step towards biomimetic cilia was made by Darnton et al., who created a bacterial carpet by attaching bacteria to a solid surface (8). Due to symmetric rotation and wea...

It has been predicted, but never confirmed, that colloidal particles in a nematic liquid crystal could be self-assembled by delocalized topological defects and entangled disclinations. We show experimentally and theoretically that colloidal dimers and 1D structures bound by entangled topological defect loops can indeed be created by locally thermally quenching a thin layer of the nematic liquid crystal around selected colloidal particles. The topological entanglement provides a strong stringlike binding, which is ten thousand times stronger compared to water-based colloids. This unique binding mechanism could be used to assemble resonator optical waveguides and robust chiral and achiral structures of topologically entangled colloids that we call colloidal wires.

We present experimental and theoretical study of colloidal interactions in quadrupolar nematic liquid crystal colloids, confined to a thin planar nematic cell. Using the laser tweezers, the particles have been positioned in the vicinity of other colloidal particles and their interactions have been determined using particle tracking video microscopy. Several types of interactions have been analyzed: (i) quadrupolar pair interaction, (ii) the interaction of an isolated quadrupole with a quadrupolar chain, and (iii) the interaction of an isolated quadrupolar colloidal particle with a two-dimensional (2D) quadrupolar crystallite. In all cases, the interactions are of the order of several 100k(B)T for 2 microm particles, which gives rise to relatively stable 2D colloidal crystals. The experimental results are compared to the predictions of Landau-de Gennes theory and we find a relatively good qualitative agreement.

Hormones are released from cells by passing through an exocytotic pore that forms after vesicle and plasma membrane fusion. In stimulated exocytosis vesicle content is discharged swiftly. Although rapid vesicle discharge has also been proposed to mediate basal secretion, this has not been studied directly. We investigated basal hormone release by preloading fluorescent peptides into single vesicles. The hormone discharge, monitored with confocal microscopy, was compared with the simultaneous loading of vesicle by FM styryl dye. In stimulated vesicles FM 4-64 (4 microM), loading and hormone discharge occurs within seconds. In contrast, in approximately 50% of spontaneously releasing vesicles, the vesicle content discharge and the FM 4-64 loading were slow (approximately 3 min). These results show that in peptide secreting neuroendocrine cells the elementary vesicle content discharge differs in basal and in stimulated exocytosis. It is proposed that the view dating back for some decades, which is that, at rest, the vesicle discharge of hormones and neurotransmitters is similar to that occurring after stimulation, needs to be extended. In addition to the classical paradigm that secretory capacity of a cell is determined by controlling the probability of occurrence of elementary exocytotic events, one will have to consider activity modulation of elementary exocytotic events as well.