Complex Fluids
Surface freezing and the kinetic pathway of the isotropic-smectic phase transition
We study the nucleation of the smectic phase out of a metastable isotropic solution and find
kinetic pathways of unexpected complexity. In order to obtain direct isotropic-smectic coexistence we prepare a
mixture of rod-like fd virus and non adsorbing polymer Dextran.
Three step kinetic pathway for the formation of colloidal membranes
step 1
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In a first step we observe nucleation of a metastable nematic tactoid in a isotropic background.
We are able to control the concentration of the rods in the droplet by changing the concentration of the polymer in
the isotropic background phase.
step 2
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Next, a quasi 2D smectic phase forms on the isotropic-nematic interface. This is a rare
example of a surface freezing phase transition. In nature most materials exhibit the opposite behavior where the
surface melts at temperatures above the bulk melting temperature. Other systems that show surface freezing
transitions are alkanes and thermotropic liquid crystals. The quasi 2D smectic phase can be prepared on a flat
isotropic-nematic interface as is shown .
step 3
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In the third and final step, the 2D surface induced smectic phase acts as a nucleation site for the
formation of isolated layers of smectic phase (colloidal membranes). Surprisingly we find that isolated colloidal
membranes nucleated at the interface grow into the isotropic phase. Fluorescence images indicate that there are no
rods present in the isotropic phase. Therefore colloidal membranes grow due to rods that diffuse from metastable
nematic phase through the surface induced smectic phase into more stable colloidal membranes.
Nucleation of Colloidal Membranes
At higher rod concentrations we observe direct nucleation of colloidal membranes out of metastable
isotropic solution. The process of membrane coalescence can be directly observed using optical microscopy.
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Movie: stickydrop.
[MOV - 2mb]
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After an equilibration time of few days very large fluctuating membranes are observed throughout
the sample. In the first movie the rods point into the plane of the screen and consequently the membrane shows no
birefringence. In the second movie the rods in the membrane lie in the plane of the screen and the images in taken
with polarization microscopy. By analyzing fluctuations shown in these movies we are able to obtain the line tension
and bending rigidity of colloidal membranes.
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Movie: membrane
from top.
[MOV - 2mb]
Movie: membrane
birefringence.
[MOV - 2mb]
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These observations are summarized in the following paper:
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