Special Seminar - From viscous to elastic sheets: Dynamics of freely floating smectic films

Thu, 07/21/2016 - 14:15
Kirsten Harth, Otto-von-Guericke Universit├Ąt Magdeburg

The dynamics of droplets and bubbles, particularly on microscopic scales, are of considerable importance in biological, environmental, and technical contexts. Soap bubbles, vesicles and components of biological cells are well known examples where the dynamic features are significantly influenced by the properties of thin membranes enclosed by fluids. Two-dimensional membrane motions couple to 3D shape transformations.

Smectic liquid crystal mesogens form phases with internal molecular layer structure. Free-standing films can be easily prepared from this class of materials. They represent simple model systems for membrane dynamics and pattern formation in a quasi two-dimensional fluid. These films are usually planar, spanned over a frame, or they can be inflated to bubbles on a support. Recently, closed microscopic shells of liquid-crystalline materials suspended in an outer fluid without contact to a solid support have been introduced and studied.
With a special technique, we prepare millimeter to centimeter sized freely floating smectic bubbles in air. Their distinct feature is the fact that any changes of their surface area are coupled to a restructuring of the layers in the membrane, leading to the formation of 'islands' or 'holes'. Islands represent spots with additional layers stacked on the film, holes are spots with reduced number of layer.

High-speed cameras are used to observe shape transformations of freely floating bubbles. In addition, bursting dynamics are recorded and compared to models.
An unpreceded cross-over from inviscid to viscous and elastic behaviour with increasing thickness of the films is found: Whereas thin bubbles behave almost like inviscid fluids, the relaxation dynamics slows down considerably for larger film thicknesses. Surface wrinkling and formation of extrusions are observed. The shape transformation related phenomena will be characterized and explained.

David Rittenhouse Laboratory - A4