Smectic Membranes in Aqueous Environment

Smectic films which are freely suspended on a solid frame in air are well established experimental model systems for the study of the structure of smectic phases, phase transitions in two dimensions, and surface and dimensionality effects in fluid systems. We could prepare freely suspended smectic films in water with a size of 1 cm2 using surfactants ensuring a strong homeotropic anchoring at the smectic/water interfaces. Since the ordering surface field at the smectic/water interface can be tuned via the surfactant coverage, smectic films in aqueous environment may expand the general range of possible studies of freely suspended smectic films. We study the stability and the thinning transitions which occur at temperatures above the volume smectic-A - isotropic transition temperature.

Fig 1: Schematic sketch of a smectic-surfactant composite membrane (left) and thinning of a smectic membrane by heating above the smectic - isotropic transition temperature (right). ΔT gives the temperature difference to the transition, the thinning is demonstrated by changes of the ellipsometric parameters Δ and Ψ (measured in transmission); the numbers near the Δ values indicate the thickness of the membrane (in units of smectic layers). Zoom Image
Fig 1: Schematic sketch of a smectic-surfactant composite membrane (left) and thinning of a smectic membrane by heating above the smectic - isotropic transition temperature (right). ΔT gives the temperature difference to the transition, the thinning is demonstrated by changes of the ellipsometric parameters Δ and Ψ (measured in transmission); the numbers near the Δ values indicate the thickness of the membrane (in units of smectic layers).

Isotropic liquid crystals might be used as carrier liquids for aqueous droplets in digital microfluidics. The smectic surface order in the isotropic temperature regime could provide a handle to control the stability of the membranes separating the water droplets. Forced electrocoalescence studies in microfluidic channels indicate an enhanced stability in the temperature range where a smectic surface order exists:

Fig. 2: Left: Micrographs of the forced electrocoalescence studies (a) Two virtually touching water phases in an isotropic liquid crystal matrix at U = 0. (b) At U = 1 V, a very thin membrane of the liquid-crystal material forms still separating the two water phases. (c) Further increasing the voltage leads to membrane rupture and coalescence of the water phases. Right: Temperature dependence of the voltages Vt (formation of the thin membrane, red triangles) and Vr (rupture of the membrane, blue circles). The yellow shading indicates the region of smectic surface order, ΔT is the temperature difference to the bulk smectic - isotropic transition. Zoom Image
Fig. 2: Left: Micrographs of the forced electrocoalescence studies (a) Two virtually touching water phases in an isotropic liquid crystal matrix at U = 0. (b) At U = 1 V, a very thin membrane of the liquid-crystal material forms still separating the two water phases. (c) Further increasing the voltage leads to membrane rupture and coalescence of the water phases. Right: Temperature dependence of the voltages V(formation of the thin membrane, red triangles) and Vr (rupture of the membrane, blue circles). The yellow shading indicates the region of smectic surface order, ΔT is the temperature difference to the bulk smectic - isotropic transition.

Find more information:

Smectic membranes in aqueous environment
Y. Iwashita, S. Herminghaus, R. Seemann, and Ch. Bahr, Phys. Rev. E 81, 051709 (2010).
DOI: 10.1103/PhysRevE.81.051709

 
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