Complex Fluids

Lamellar melting in a temperature sensitive colloidal lamellar phase

In collaboration with Ahmed Alsayed and Arjun Yodh (University of Pennsylvania)

Because of time and length scales colloidal system are ideal systems to study nucleation processes. In contrast to freezing, melting transitions are more difficult to study in a colloidal suspension because the phase behavior of most colloidal systems is temperature independent. Once a stable ordered phase is obtained the only way to induce a melting transition is by changing a colloidal concentration which is a challenging experimental task. To circumvent this problem and study the melting behavior of a lamellar phase we prepare a mixture of rod-like fd virus and thermosensitive isopropylacrylamide (NIPA) polymer. In this system changing the temperature effectively controls the rod concentration.

Movie: smectic drop melting. [MOV - 2mb]

With increasing temperature we observe melting of a smectic droplet into a nematic droplet as shown in the movie. The proposed mechanism for the melting of the smectic droplet is as follows. With increasing temperature the osmotic pressure of the semi-dilute NIPA solution decreases due to increased importance of monomer-monomer attraction. As a result water from polymer-rich isotropic phase flows into rod-rich smectic droplet which in turn dilutes the rod concentration within the droplet. This can result in the melting of the smectic droplet as shown in the movie.