Boris Khusid

Deterministic and stochastic behaviour of non-Brownian spheres in sheared suspensions

The dynamics of macroscopically homogeneous sheared suspensions of neutrally buoyant, non-Brownian spheres is investigated in the limit of vanishingly small Reynolds numbers using Stokesian dynamics. We show that the complex dynamics of sheared suspensions can be characterized as a chaotic motion in phase space and determine the dependence of the largest Lyapunov exponent on the volume fraction ϕ. We also offer evidence that the chaotic motion is responsible for the loss of memory in the evolution of the system and demonstrate this loss of correlation in phase space. The loss of memory at the microscopic level of individual particles is also shown in terms of the autocorrelation functions for the two transverse velocity components. Moreover, a negative correlation in the transverse particle velocities is seen to exist at the lower concentrations, an effect which we explain on the basis of the dynamics of two isolated spheres undergoing simple shear. In addition, we calculate the probability distribution function of the transverse velocity fluctuations and observe, with increasing ϕ, a transition from exponential to Gaussian distributions. The simulations include a non-hydrodynamic repulsive interaction between the spheres which qualitatively models the effects of surface roughness and other irreversible effects, such as residual Brownian displacements, that become particularly important whenever pairs of spheres are nearly touching. We investigate, for very dilute suspensions, the effects of such a non-hydrodynamic interparticle force on the scaling of the particle tracer diffusion coefficients D y and D z , respectively, along and normal to the plane of shear, and show that, when this force is very short-ranged, both are proportional to ϕ 2 as ϕ → 0. In contrast, when the range of the non-hydrodynamic interaction is increased, we observe a crossover in the dependence of D y on ϕ, from ϕ 2 to ϕ as ϕ → 0. We also estimate that a similar crossover exists for D z but at a value of ϕ one order of magnitude lower than that which we were able to reach in our simulations.

Combined field-induced dielectrophoresis and phase separation for manipulating particles in microfluidics

Experiments were conducted in microfluidics equipped with dielectrophoretic gates arranged perpendicular to the flow. Under the action of a high-gradient ac field and shear, flowing suspensions were found to undergo a phase separation and to form a distinct front between the regions enriched with and depleted of particles. We demonstrate that this many-body phenomenon, which originates from interparticle electrical interactions, provides a method for concentrating particles in focused regions and for separating biological and nonbiological materials. The evolution of the particle patterns formation is well described by a proposed electrohydrodynamic model.

Microstructure and velocity fluctuations in sheared suspensions

The velocity fluctuations present in macroscopically homogeneous suspensions of neutrally buoyant, non-Brownian spheres undergoing simple shear flow, and their dependence on the microstructure developed by the suspensions, are investigated in the limit of vanishingly small Reynolds numbers using Stokesian dynamics simulations. We show that, in the dilute limit, the standard deviation of the velocity fluctuations is proportional to the volume fraction, in both the transverse and the flow directions, and that a theoretical prediction, which considers only for the hydrodynamic interactions between isolated pairs of spheres, is in good agreement with the numerical results at low concentrations. We also simulate the velocity fluctuations that would result from a random hard-sphere distribution of spheres in simple shear flow, and thereby investigate the effects of the microstructure on the velocity fluctuations. Analogous results are discussed for the fluctuations in the angular velocity of the suspended spheres. In addition, we present the probability density functions for all the linear and angular velocity components, and for three different concentrations, showing a transition from a Gaussian to an Exponential and finally to a Stretched Exponential functional form as the volume fraction is decreased. We also show that, although the pair distribution function recovers its fore-aft symmetry in dilute suspensions, it remains anisotropic and that this anisotropy can be accurately described by assuming the complete absence of any permanent doublets of spheres. We finally present a simple correction to the analysis of laser-Doppler velocimetry measurements.

Particle segregation in suspensions subject to high-gradient ac electric fields

An experimental and theoretical study is described dealing with the dielectrophoretic motion of individual particles in a static as well as in a flowing suspension subject to high-gradient ac electric fields. The experiments were performed on very dilute suspensions of neutrally buoyant hollow ceramic spheres in a specially designed device in which the electric-field lines and the dielectrophoretic force were along the plane perpendicular to the streamlines of the main flow. Upon application of a high-gradient field (∼several kV/mm) to a quiescent suspension, the particles were found to move away from the electrodes and then to concentrate above the grounded electrodes, forming a distinct boundary between the clean fluid and the remaining suspension. This same field, when applied to a flowing suspension, caused the particles to concentrate within thin stripes parallel to the flow above the grounded electrodes and to travel with the suspending fluid within these stripes. The theoretical model for the parti...

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.

Inhibition and Promotion of Copper Corrosion by CTAB in a Microreactor System

We report on an optical microscopy technique for the analysis of corrosion kinetics of metal thin films in microreactor systems and use it to study the role of cetyltrimethylammonium bromide surfactant as a corrosion inhibitor in a copper-gold galvanic coplanar microsystem. A minimum in the dissolution rate of copper is observed when the surfactant concentration is approximately 0.8 mM. To explain why the inhibitory role of the surfactant does not extend to higher concentrations, we use zero resistance ammetry with separated half cells and show that while the surfactant inhibits cathodic reactions on gold, it also promotes the corrosion of copper because of the catalytic action of bromide counterions. These two competing processes lead to the observed minimum in the dissolution rate.

Wetting and particle adsorption in nanoflows

Molecular dynamics simulations are used to study the behavior of closely fitting spherical and ellipsoidal particles moving through a fluid-filled cylinder at nanometer scales. The particle, the cylinder wall, and the fluid solvent are all treated as atomic systems, and special attention is given to the effects of varying the wetting properties of the fluid. Although the modification of the solid-fluid interaction leads to significant changes in the microstructure of the fluid, its transport properties are found to be the same as in bulk. Independently of the shape and the relative size of the particle, we find two distinct regimes as a function of the degree of wetting, with a sharp transition between them. In the case of a highly wetting suspending fluid, the particle moves through the cylinder with an average axial velocity in agreement with that obtained from the solution of the continuum Stokes equations. In contrast, in the case of less-wetting fluids, only the early time motion of the particle is c...

Positive dielectrophoresis and heterogeneous aggregation in high-gradient ac electric fields

Experiments were conducted in a parallel-plate channel in which an extremely dilute suspension of heavy, positively polarized spheres was exposed to an ac electric field under conditions such that the field lines were arranged in the channel cross section perpendicular to the streamlines of the main flow. To reduce the effects of the gravitational settling of the particles, the channel was slowly rotated around a horizontal axis. Following the application of a high-gradient strong ac field (∼ several kV/mm), the particles were found to move towards both the high-voltage and grounded electrodes and to form arrays of “bristles” along their edges. The process was also modeled theoretically by computing the trajectories of individual particles under the action of dielectrophoretic, viscous, and gravitational forces and under conditions of negligibly small particle Reynolds numbers. The model calculations required no fitting parameters because the particle polarizability was determined independently by measuri...

Dynamical clustering of counterions on flexible polyelectrolytes.

Molecular dynamics simulations are used to study the spatiotemporal dynamics of charge fluctuations around a polyelectrolyte molecule at charge densities above and below the classic counterion condensation threshold. Surprisingly, the counterions form weakly interacting clusters which exhibit slowly decaying short range orientational order. Local charge fluctuations create energy fluctuations at the order of k_(B)T that is sufficient to affect the polyelectrolyte interaction with an approaching ligand molecule. The predictions of the classical theory appear to be appropriate only over much longer time scales.

Limitations on the scale of an electrode array for trapping particles in microfluidics by positive dielectrophoresis

Experiments were performed on dilute suspensions of polarized particles in microfluidics with electrodes of different sizes arranged parallel and perpendicular to the flow. We identified the conditions under which the interparticle electric interactions do not affect the particle trapping in the high-field strength regions. In particular, there exists a lower bound on the scale of the microelectrodes, below which the expected improvement in positioning the particles in the preselected locations cannot be attained.