Local phase transitions in driven colloidal suspensions

Using dynamical density functional theory and Brownian dynamics simulations, we investigate the influence of a driven tracer particle on the density distribution of a colloidal suspension at a thermodynamic state point close to the liquid side of the binodal. In bulk systems, we find that a localised region of the colloid-poor phase, a ‘cavitation bubble’, forms behind the moving tracer. The extent of the cavitation bubble is investigated as a function of both the size and velocity of the tracer. The addition of a confining boundary enables us to investigate the interaction between the local phase instability at the substrate and that at the particle surface. When both the substrate and tracer interact repulsively with the colloids we observe the formation of a colloid-poor bridge between the substrate and the tracer. When a shear flow is applied parallel to the substrate the bridge becomes distorted and, at sufficiently high shear-rates, disconnects from the substrate to form a cavitation bubble.     

外部信号驱动下胶原粒子在谐振势场中的协作效应

基于朗之万方程研究了一个运动在简谐势场中胶原粒子的热力学行为,给出了其在周期信号驱动下的有效温度和耗散功的解析表达式.结果表明：外部周期搅拌能够聚合环境热能传递给胶原粒子,使其有效温度大幅度增加,这是势场、外部信号及噪声的协作机理所引起;但是在共振条件下,外界所作的耗散功也达到极大. 关键词： 耗散功 有效温度 周期信号 协作效应     

Influence of colloidal particle transfer on the quality of self-assembling colloidal photonic crystal under confined condition

The relationship between colloidal particle transfer and the quality of colloidal photonic crystal(CPC) is investigated by comparing colloidal particle self-assembling under the vertical channel(VC) and horizontal channel(HC) conditions.Both the theoretical analyses and the experimental measurements indicate that crystal quality depends on the stability of mass transfer.For the VC,colloidal particle transfer takes place in a stable laminar flow,which is conducive to forming high-quality crystal.In contrast,it happens in an unstable turbulent flow for the HC.Crystals with cracks and an uneven surface formed under the HC condition can be seen from the images of a field emission scanning electron microscope(SEM) and a three-dimensional(3D) laser scanning microscope(LSM),respectively.     

Fresnel light drag in a coherently driven moving medium

We theoretically study how the phase of a light plane wave propagating in a resonant medium under electromagnetically induced transparency (EIT) is affected by the uniform motion of the medium. For cuprous oxide (Cu2O), where EIT can be implemented through a typical pump-probe configuration, the resonant probe beam experiences a phase shift (Fresnel-Fizeau effect) that may vary over a wide range of values, positive or negative, and even vanishing, due to the combined effects of the strong frequency dispersion and anisotropy both induced by the pump. The use of such a coherently driven dragging medium may improve by at least 1 order of magnitude the sensitivity at low velocity in optical drag experiments.     

Line defect dynamics around a colloidal particle

We study experimentally the dynamics of a topological defect located around a colloidal particle suspended in a thermotropic nematic liquid crystal. The considered defect consists of a disclination loop encircling the particle at the equator. Under specific conditions, it is shown that this disclination continuously shrinks to a hedgehog defect located in the immediate vicinity of the particle. This phenomenon corresponds to a transition between an elastic quadrupolar configuration and an elastic dipolar configuration. We performed a basic numerical calculation to get an estimate of the dissipated energy during the transition; we compare the results with theoretical predictions that describe the elastic energy of particles surrounded by defects. Received 21 December 2001     

Adsorption phenomena in the transport of a colloidal particle through a nanochannel containing a partially wetting fluid

Using molecular dynamics simulations, we study the motion of a closely fitting nanometer-size solid sphere in a fluid-filled cylindrical nanochannel at low Reynolds numbers. At early times, when the particle is close to the middle of the tube, its velocity is in agreement with continuum calculations, despite large thermal fluctuations. At later times, partially wetting fluids exhibit novel adsorption phenomena: the sphere meanders away from the center of the tube and adsorbs onto the wall, and subsequently either sticks to the wall and remains motionless on average, or separates slightly from the tube wall and then either slips parallel to the mean flow or executes an intermittent stick-slip motion.     

Proposal for a particle accelerator driven by a far-infrared free-electron laser

We propose a scheme of particle accelerators driven by far-infrared free-electron lasers on the basis of oversized periodically or helically corrugated hollow waveguides with cooled walls of high-T_c superconductors. In this context, we discuss the basic principles of particle acceleration in periodic and helical hollow waveguides, the relevant modes and fields of these waveguides, attenuation and damage threshold in the waveguides assuming walls made of standard metals as well as high-T_c superconductors, and the energy gain of particles for two modes of coupling laser radiation into the waveguides.     

Levitation, lift, and bidirectional motion of colloidal particles in an electrically driven nematic liquid crystal

We study electric-field-induced dynamics of colloids in a nematic cell, experimentally and by computer simulations. Solid particles in the nematic bulk create director distortions of dipolar type. Elastic repulsion from the walls keeps the particles in the middle of cell. The ac electric field reorients the dipoles and lifts them to top or bottom, depending on dipole orientation. Once near the walls, the colloids are carried along two antiparallel horizontal directions by nematic backflow. Computer simulations of the backflow agree with the experiment.     

Low-energy modes and Debye behavior in a colloidal crystal

We study the vibrational spectrum and the low-energy modes of a three-dimensional colloidal crystal using confocal microscopy. This is done in a two-dimensional cut through a three-dimensional crystal. We find that the observed density of states is incompatible with the standard Debye form in either two or three dimensions. These results are confirmed by numerical simulations. We show that an effective theory for the projections of the modes onto the two-dimensional cut describes the experimental and simulation data in a satisfactory way.     

Shear thinning and local melting of colloidal crystals

Phenomena such as shear thinning and thickening, occurring when complex materials are exposed to external forces, are generally believed to be closely connected to changes in the microstructure. Here, we establish a direct and quantitative relation between shear thinning in a colloidal crystal and the surface area of the locally melted region by dragging a probe particle through the crystal using optical tweezing. We show that shear thinning originates from the nonlinear dependence of the locally melted surface area on the drag velocity. Our observations provide unprecedented quantitative evidence for the intimate relation between mechanical properties and underlying changes in microscopic structure.     

Freezing and melting of a colloidal adsorbate on a 1D quasicrystalline substrate

Using Monte Carlo simulations and an extended Landau-Alexander-McTague theory, we demonstrate that colloids in a one-dimensional quasicrystalline potential order in triangular and rhombic-alpha crystalline phases. Increasing the strength of the potential further, a new type of light-induced melting is discovered that has its origin in the nonperiodicity of the potential. In contrast to reentrant melting in periodic potentials, the quasicrystalline potential melts the crystalline phases even when they already exist at zero potential.     

Work distribution function for a Brownian particle driven by a nonconservative force

We derive the distribution function of work performed by a harmonic force acting on a uniformly dragged Brownian particle subjected to a rotational torque. Following the Onsager and Machlup’s functional integral approach, we obtain the transition probability of finding the Brownian particle at a particular position at time t given that it started the journey from a specific location at an earlier time. The difference between the forward and the time-reversed form of the generalized Onsager-Machlup’s Lagrangian is identified as the rate of medium entropy production which further helps us develop the stochastic thermodynamics formalism for our model. The probability distribution for the work done by the harmonic trap is evaluated for an equilibrium initial condition. Although this distribution has a Gaussian form, it is found that the distribution does not satisfy the conventional work fluctuation theorem.     

Thermodynamics of a colloidal particle in a time-dependent nonharmonic potential

We study the motion of an overdamped colloidal particle in a time-dependent nonharmonic potential. We demonstrate the first lawlike balance between applied work, exchanged heat, and internal energy on the level of a single trajectory. The observed distribution of applied work is distinctly non-Gaussian in good agreement with numerical calculations. Both the Jarzynski relation and a detailed fluctuation theorem are verified with good accuracy.     

Elastic behavior of a two-dimensional crystal near melting

Using positional data from video microscopy, we determine the elastic moduli of two-dimensional colloidal crystals as a function of temperature. The moduli are extracted from the wave-vector-dependent normal-mode spring constants in the limit q-->0 and are compared to the renormalized Young's modulus of the Kosterlitz-Thouless-Halperin-Nelson-Young theory. An essential element of this theory is the universal prediction that Young's modulus must approach 16 pi at the melting temperature. This is indeed observed in our experiment.     

The molecular dynamics of a naphthalene crystal near melting

It is suggested that atom-atom potential functions become more appropriate for molecular dynamics calculations for systems as they approach melting. A simulation for naphthalene shows the molecular rate of reorientation about the axis of greatest inertia to be approaching 100 MHz within 20 K from melting. Self-correlations for this motion are highly significant, though neighbour correlations appear to be random in this case. Such behaviour is contrasted with plastic crystal behaviour where neighbour correlations are high, and it is suggested that this characterises the difference between a true crystal and a plastic crystal. For naphthalene this result contrasts with an experimental conclusion where the motion about the axis of least inertia is though to be responsible for the onset of melting.     

Nonlinear effects in the electrophoresis of a spherical colloidal particle

The reduced electrophoretic mobility-reduced zeta potential relationship for a charged macroparticle is shown to be nonuniversal and to be highly nonlinear. In agreement with experimental results, a mobility reversal due to the macroion's charge inversion and a nonlinear dependence of the mobility on salt concentration is obtained.