(504g) Particle Focusing in Microfluidic Flows

Particle focusing and separation in microfluidic flows and devices has been an area of active investigation because of its broad relevance for point of care testing in biomedical and environmental applications that involve counting, sorting, and detecting or trapping particles or cells. An important example is microfluidic flow cytometry which requires a single-file stream of particles or cells to allow individual optical interrogation. In this talk, we present an analysis of inertial and steric particle focusing mechanisms for particles encountering an orifice or slit pore. Focusing depends on the ratio of particle size to pore size and a suitably defined Stokes number that characterizes particle inertia. Pressure-driven and electroosmotic flows are considered. Exact boundary integral simulations are presented using a Green’s function that incorporates the no-slip boundary conditions on a perforated boundary. A critical Stokes number is reported corresponding to optimal focusing. Exponential focusing is predicted for any Stokes number. An analytic theory is developed for particles much smaller than the pore size. Steric focusing results from contact interactions between particles and pore edges. Both mechanisms can be used to sort particles by size or inertia.