Abstract
Gravity can affect colloidal suspensions since for micrometer-sized particles gravitational and thermal energies can be comparable over vertical length scales of a few millimeters. In mixtures, each species possesses a different buoyant mass, which can make experimental results counter-intuitive and difficult to interpret. Here, we revisit from a theoretical perspective iconic sedimentation-diffusion-equilibrium experiments on colloidal plate-rod mixtures by van der Kooij and Lekkerkerker. We reproduce their findings, including the observation of five different mesophases in a single cuvette. Using sedimentation path theory, we incorporate gravity into a microscopic theory for the bulk of a plate-rod mixture. We also show how to disentangle the effects of gravity from sedimentation experiments to obtain the bulk behavior and make predictions that can be experimentally tested. These include changes in the sequence by altering the sample height. We demonstrate that both buoyant mass ratio and sample height form control parameters to study bulk phase behavior.
Highlights
Gravity can affect colloidal suspensions since for micrometer-sized particles gravitational and thermal energies can be comparable over vertical length scales of a few millimeters
Sedimentation experiments, in which a colloidal suspension is placed in a cuvette under the influence of gravity, are ideal candidates to study the bulk behavior of colloidal systems
Gravity can play a major role in the colloidal realm and can only be neglected if the relevant length scales of the experiment are much smaller than the colloidal gravitational length; the latter is the ratio between the thermal energy and the buoyant force acting on a single particle
Summary
Gravity can affect colloidal suspensions since for micrometer-sized particles gravitational and thermal energies can be comparable over vertical length scales of a few millimeters. We reproduce their findings, including the observation of five different mesophases in a single cuvette. Since in many colloidal systems the gravitational length is smaller than the typical height of a cuvette, even sedimentation experiments with samples of few millimeters in height provide in-depth insight into the equation of state and bulk phase phenomenology. Counterintuitive and complex phenomenology arises, making it difficult to draw conclusions about bulk behavior In their iconic experiments on plate-rod mixtures, van der Kooij and Lekkerkerker[22,23], found hitherto unexpected and rich phenomenology of colloidal mixtures. The two nematic layers correspond to different bulk phases rich in either rods (top) or plates (bottom)
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