(179e) Separation of Particles Using Depletion Forces in a Packed Column

Attractive depletion forces were used to enhance the separation of micron-sized colloidal particles flowing through a packed bed.  The dispersion consisted of a binary mixture of negatively charged polystyrene sulfonate (PS) particles (diameter of 0.82 and 1.5 micron) in a solution of negatively-charged, 7 nm diameter silica nanoparticles, and the column packing was 0.5 mm diameter silica beads.  The experiments consisted of a short-pulse injection of the PS particles into the nanoparticle dispersion at the inlet of the packed column. The nanoparticles created an attractive depletion force between PS particles and the column packing which reduced the rate at which the microparticles passed through the column.  Because the magnitude of the depletion force scales with the particle size, the impact of the force on the rate of transport varied with size.  The degree of separation was quantified by analyzing particle breakthrough curves and measuring the concentration of particles in the collected effluent. Control experiments without any nanoparticles were done using a carrier fluid with an equivalent ionic strength and pH as the solution of nanoparticles.  These experiments illustrate a novel application of depletion forces for particle separations.