Session HH: Suspensions III

Dynamics of suspensions of pH-responsive hydrogel colloids

Colloidal hydrogel particles have attracted interest as building blocks for chemical sensors, photonic crystals and as drug delivery vehicles. In addition, they are interesting model systems to study the phase behavior of colloidal particles with soft interaction potentials. The most commonly used pNIPAm hydrogels are temperature sensitive, showing a swelling-deswelling transition around 30 degrees Celcius; by including acrylic acid comonomer, one can obtain pNIPAm-co-AAc hydrogels that are also pH responsive. For this work, we have investigated the dynamics of swelling and deswelling of these stimuli responsive colloids in both diluted and concentrated suspensions via particle tracking video-microscopy in a transparent dialysis cell. The device allows us to change the solvent composition (e.g. pH) in a controlled manner while simultaneously tracking the motion of hydrogel particles. In dilute suspensions, we have studied the swelling-deswelling response of hydrogels of different sizes and varying AAc and cross-linker contents to elucidate the kinetics of the microstructural rearrangements of the hydrogel. In concentrated suspensions, the pH-induced particle expansion causes transitions between fluid, glassy and crystalline phases. Data will be presented on the dynamics of the observed phase behavior, in particular crystal growth and jamming.   

The Osmotic Motor

We propose a model for self-propulsion of a colloidal particle -- the osmotic motor -- immersed in a dispersion of colloidal `bath' particles. The osmotic motor is propelled by a chemical reaction that consumes bath particles over a portion of its surface. The non-equilibrium microstructure of bath particles induced by the surface reaction creates an `osmotic pressure' imbalance on the motor's surface causing it to move to regions of lower bath particle concentration. The osmotic motor's behavior is calculated for two scenarios: one in which the motor is held fixed and a second where it is free to move. The departure of the bath particle concentration from equilibrium is characterized by the Damk\"{o}hler number \textit{Da}: the ratio between the surface reaction velocity and the diffusion velocity. The computed microstructure is employed to calculate the driving force of the motor, from which the self-induced osmotic velocity is determined via application of Stokes drag law. For small departures from equilibrium (\textit{Da }$<<$ 1), the self-propulsion is determined by the reaction velocity. In the large \textit{Da} limit the surface reaction dominates over diffusion and the osmotic velocity cannot be greater than the speed of bath particles that are about to react. The implications of these features for different bath particle volume fractions and particle sizes are discussed. Theoretical predictions are compared with Brownian Dynamics simulations.   

Developing on-chip methods for the manipulation of particles using conventional and traveling wave dielectrophoresis

We present the design of a microfluidic device, electrical circuits and an ac electrical power supply capable of combining both conventional (DEP) and traveling wave dielectrophoresis (tw-DEP) in a single microchip for the consecutive separation and manipulation of suspended particles according to their electrical properties. The microfluidic device is fabricated using two microchips, having interdigitated microelectrodes, and the microchips were aligned parallel to each other to form the fluidic chamber. The electrode width and inter-electrode spacing for both chips are 20-microns. In contrast to previous studies on traveling-wave dielectrophoresis where the amplitude of the applied voltage was limited to 20Vp-p, we have assembled an advance electrical power supply, using a dual channel function generator and two voltage amplifiers, capable of producing four sinusoidal signals, with each signal shifted in phase from the previous by 90 degrees, with voltage amplitude of up to 200 V (peak-to-peak) and frequency of up to 250 kHz. During this talk, we will present the operating principle of the device for different electric field configurations, for dielectrophoresis and traveling-wave dielectrophoresis, along with the experimental results on suspensions of 3 micrometer diameter latex particles dispersed in an aqueous medium.   

Electric field driven formation of particle concentration fronts in suspensions

A distinct front, separating regions enriched with and depleted of particles, was recently observed in suspensions subjected to high-gradient ac electric fields and a set of equations for the field-driven suspension flow, containing no fitting parameters, were developed [Kumar et. al. 2004, Phys. Rev. E 69, 021402-1-10; Bennett et. al. 2003, Appl. Phys. Lett. 83, 4866-4668]. Although the numerical solutions of these equations were found to be quantitatively consistent with the experimental observations, they did not provide sufficient information for elucidating the mechanism of the front formation due to the complexity of the equations. Here, we examine analytically the dynamics of the concentration front formation and propagation by considering these equations for the special case in which they can be simplified and then reduced, via a similarity transformation, to ordinary differential equations. We establish the existence of shock solutions to these equations and determine the location of the concentration front as well as the dependence of the front velocity on the bulk particle concentration of the suspension. In particular, we demonstrate that the appearance of the front is caused by the rapid local growth of the suspension viscosity due to the field-driven particle accumulation in a certain area of the domain.   

Scalings and Stability in Monodisperse Fluidized Beds

We have measured the concentration and fluctuation profiles to investigate steady state sedimentation of nearly monodisperse ($\sigma_a/a\le 1.6\%$) fluidized beds over a wide range of particle sizes $a$. In terms of the normalized particle column height $H^*\equiv H/a$, we find that taller columns are more stratified, and exhibit larger fluctuations, than shorter columns. Operating at a single average volume fraction $\phi_0$, we find scaling relations for the concentration at the top interface, $\phi_{top}=\phi_0e^{-H^*/3711}$, the velocity fluctuations, $\sigma_v(z)/v_0=\sqrt{H^*}\alpha(z)$, and the correlation lengths, $\xi(z)=a\beta(z)$. Finally, we develop a new advection-diffusion model that describes the observed bed stability.   

Segregation and mixing in bidisperse liquid-fluidized suspension.

We study experimentally the fluidization of a bidisperse suspension of macroscopic particles (150-160 microns and 180-200 microns glass beads), at low Reynolds number. With the help of an acoustic scanner, the measurement of the sound propagation (velocity and attenuation) at an appropriate frequency (3MHz), is continuously recorded along the bed. Those measurements are linked to the concentrations of the particles, and provide the composition, in time, of the suspension along the vertical axis. In our system, one may expect a segregation process induced by the different settling velocities, which should result in a stationary segregated state: a monodisperse suspension of small particles fluidized on top of a monodisperse suspension of large particles, with a transition zone enlarged by the mixing of particles due to hydrodydamic dispersion. However, no stationary state has been observed in our experimental system. In the investigated range of low injection rates, our fluidized bidisperse suspension exhibits oscillations between segregated and homogeneous states.