Acoustic aging of colloidal gels

Colloidal gels undergo aging through thermal motion that drive aggregation, coarsening, and phase separation. Conventional aging acceleration methods, including heat and centrifugation, often modify interparticle forces or disrupt microstructural integrity. Here, we show that low-power ultrasound is able to accelerate aging in model colloidal dispersions while preserving their intrinsic destabilization mechanisms. We synthesize weak colloidal gels using fluorescent colloidal polymethyl methacrylate particles (diameter 2a = 1 µm ± 3%) dispersed in a cyclohexyl bromide–decalin (66:34 v/v) solvent mixture containing a polystyrene depletant (concentration c/c* = 1.15, radius of gyration R_g = 45 nm). The electrostatic charges are screened by addition of 4 mM of tetrabutyl ammonium chloride. After equilibration, the colloidal gels are subjected to continuous 20 kHz acoustic excitation with an acoustic pressure of 47 kPa and a total input power of 0.6 W. We find that the acoustic exposure increases the rate of macroscopic phase separation with an acceleration factor of 1.45. Confocal laser scanning microscopy shows that the acoustically stimulated gels contain larger clusters and thicker strands, quantified by contact number and radial distribution analyses. Under acoustic perturbation, the sequence of microstructural events observed is identical to that of naturally aged samples but shifted towards a shorter timescale, suggesting that low-power acoustics promote aging through increasing the diffusion kinetics of individual colloids without increasing the bulk temperature.

Keywords: colloidal gels, aging, acceleration, low-power acoustics, image processing, confocal microscopy