Physics Colloquium
Friday, December 3rd, 2004, 4:00 P.M.

Gerald Fuller

Chemical Engineering
Stanford University

Complex Fluid Interfaces: Rheology in Two Dimensions

Complex fluid interfaces, like their bulk counterparts, are richin structure and are highly nonlinear in their response to weak external forces. These are deformable interfaces where amphiphilicmolecules and colloidal particles collect, and render the systems rheologicallycomplex. They are found all around us in the form of biologicalfilms and in a great many consumer products. Examples from biology include the cell membrane and the alveoli of our lungs; industrial applications include emulsions and foams, where surface-active additives are used as stabilizers. These mobile, two-dimensional systems require new experimental methods to characterize their rheologicalresponse, and the connection to interfacial microstructure. This lecture introduces mechanical and optical methods that have been devised for these purposes and highlight their application to the following problems:

• Self-assembled monolayers(fatty acids/alcohols and phospholipids). Insoluble monolayersof asymmetric, low molecular weight molecules can arrange themselves to form remarkable structures and intricate phase diagrams. Experiments are presented that demonstrate transitions from linear, Newtonian interfaces to behavior similar to smecticliquid crystals.

• Polymeric amphiphiles. Polymer chains are often amphiphilicand cause interfaces to display non-Newtonian responses such as shear thinning surface viscosities and normal stress differences. These nonlinearities lead to significant departures in interfacial flow phenomena.

• Biological thin films. Most biological films come in the form ofwell-ordered monolayersand bilayers, and show the phenomena described in the first class of examples. Specific examples presented in this lecture include 2D electrophoresis of DNA and the reaction of lung surfactants to environmental pathogens.

• Particle stabilized emulsions. Colloidal particles are stronglyattracted to liquid-liquid interfaces and are used as stabilizers in "Pickering emulsions". The recent results in the rheology, adhesion and stability of particleladen interfaces are presented.

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