Fig. 2: Effect of gravity on colloidal particles.
Schematic representation of some key effects of gravity on small (colloidal) or large (granular) particles in a fluid. a In the absence of gravity, thermal fluctuations lead to a uniform distribution of colloidal particles. b For granular particles this process can be prohibitively slow, and hence injection of mechanical energy is required for the system to remain in a fluid state. c Gravity promotes a net flux of particles (sedimentation) along the direction of gravity. d If the mass distribution within each particle is not spherically symmetric, gravity results in a net torque on the particles, which can promote their orientational ordering. e For a diluted colloidal suspension, the steady-state distribution of particles in the presence of gravity is not homogeneous along the direction of gravity but corresponds to an exponential concentration profile. f In a granular medium, the particles densely accumulate at the bottom of the container, where they minimize their gravitational potential energy and form load-bearing frictional contacts.
