Stimuli-Responsive Colloidal Gate for Active Modulation of Fluid Flow in Packed Beds

We present pH-responsive colloidal paste based on brushes of poly(acrylic acid) (PAA) grafted on silica microparticles (∼4 μm) that actively modulates bulk fluid flow in packed bed columns of the colloid paste. pH-dependent charge and conformational changes of the PAA brushes in aqueous 10 mM NaNO₃ solutions (pH 2-10) give an 8-fold change in flow rate (0.04-0.32 mL/min) through dynamic particle rearrangement in the paste, offering potential for flow rate modulation by pH. The colloid paste is a onsive "colloidal flow gate" that actively changes its permeability based on the aqueous solution acidity. We characterize the colloidal paste permeability by measuring pH variations of the flow rate through the paste.