Liquid Crystal Foams: Formation and Coarsening

Abstract

Coarsening in foams made from the pure liquid crystal, 8CB, has been studied. The foam was made in the nematic phase ($\textrm{T} = 35 ^{\circ}\textrm{C}$) by bubbling nitrogen through the pure liquid crystal. The coarsening behavior was investigated at three temperatures; at $\textrm{T} = 22 ^{\circ}\textrm{C}$ and $33^{\circ}\textrm{C}$ in the smectic phase and at $\textrm{T} = 34^{\circ}\textrm{C}$ in the nematic phase. In smectic and nematic phases the mean bubble radius $<R>$ has been measured as a function of time $<R > \sim t^λ$. In classical wet soap foams the growth exponent is typically $λ\approx 0.33$ where coarsening is by gas diffusion from bubbles with high curvature to bubbles with low curvature. In liquid crystal foams a growth exponent, $λ= 0.20 \pm 0.05$ is observed. This may be explained by the presence of defects at the surface of the bubbles which slow down the coarsening behaviour. This growth exponent can be observed in both nematic and smectic phases. At higher temperatures typically $>35^{\circ}\textrm{C}$ coalescence dominates the coarsening behaviour. In the isotropic state, $>41.5^{\circ}\textrm{C}$, the foam is rapidly unstable.