Aging of colloidal gels in microgravity

TL;DR

This study investigates how colloidal gels age in microgravity, revealing slowed particle dynamics, structural evolution, and the influence of attractive forces, using space-based microscopy and ground comparisons.

Contribution

It provides new insights into colloidal gel aging in microgravity, including particle dynamics, structural changes, and the relationship between 2D and 3D gel structures.

Findings

Particle dynamics slow down with gel aging.

Stronger attractions lead to thicker gel strands.

Microgravity experiments show power-law decay in particle motion.

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.