Wednesday, November 10, 2010: 10:15 AM
Canyon A (Hilton)
Recent studies have shown that a surface chemical reaction could induce the motion of rigid colloidal particles. However, no prior work has described the potential of particles that could permeate fluid but no solutes in applications such as lab-on-a-chip and drug delivery carriers. This work adds some light into these systems by proposing a model for the catalytically-driven motion of semipermeable particles (e.g., non-motile microorganisms and vesicles) immersed in a Newtonian fluid. For simplicity, we assume that a first order consumption reaction, which could be caused by enzymes or catalytic materials, is located on half of the outer surface of the membrane. In fact, this 'non-uniformity' criteria for the reaction is essential for directed motion. Initially, the reactant concentration is the same inside and outside the particle, thus preventing any osmotic flow. When the reaction is active, an imbalance in the osmotic pressure is created about the outer surface of the particle that induces the motion of the particle towards low reactant concentration regions by a mechanism known as osmophoresis. The fluid faced by the catalytic side permeates toward the inside of the particle due to the low reactant concentration region created by the particle. Conservation of fluid mass inside the particle results in permeation of fluid from inside to outside the particle throughout the inert membrane. The results show that the velocity of the particle compared to the diffusive speed of surrounding reactants—defined as a Péclet number—is a function of the permeability of the membrane, a 'characteristic' osmotic velocity due to the presence of reactants, and the Damköhler number—a measurement of how fast the reaction is compared to the diffusion of the reactants. Decreasing the permeability of the membrane results in slower velocities independently of the Damköhler number and the reactant concentration. If the semipermeable particle moves too fast, the permeating fluid will drag reactants to the catalytic side, and therefore, reduces the imbalance in osmotic pressure which causes propulsion in the first place.