Aging of colloidal gels in microgravity

Abstract: We study the aging of colloidal gels using light microscopy movies of depletion gels from the International Space Station. Under such microgravity conditions, we observe a slowdown in particle dynamics consistent with gel aging. Stronger attractive forces promote the formation of thicker gel strands over time. The samples are bidisperse, composed of particles with a size ratio 1.2. Larger particles experience stronger depletion forces, which lead to a large first-neighbor peak in the pair correlation function $g(r)$ due to the prevalence of large-large particle contacts. As the gel ages, small mobile particles are incorporated into the gel structure. The changes in gel structure correlate with a slow power-law decay in particle motion, observed over nearly two orders of magnitude of time scales in microgravity experiments. Additionally, through complementary ground-based experiments, we compare two-dimensional (2D) and three-dimensional (3D) images of depletion colloidal gels. While microgravity gel data are limited to 2D projections, ground-based data establish a correspondence between the 2D and 3D $g(r)$ peak heights. Our results provide new insights into how colloidal gels age in the absence of gravitational collapse.