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Complex Fluids and Complex Fluid Interfaces
Orientation Dynamics and Complex Liquids
The processing of polymeric and other complex materials alters their microstructure through orientation and deformation of their constitutive elements. In the case of polymeric liquids, it is of interest to obtain in situ measurements of segmental orientation and optical methods have proven to be an excellent means of acquiring this information. Research in our laboratory has resulted in a number of techniques in optical rheometry such as high-speed polarimetry (birefringence and dichroism) and various microscopy methods (fluorescence, phase contrast, and atomic force microscopy).
Elegant flow-processing techniques have been developed to produce organized, biocompatible structures for applications in tissue engineering. Collagen, protein commonly found in load-bearing tissues, possesses the unique ability to organize into complex ordered structures due to its liquid crystalline properties. The result is an oriented substrate of collagen capable of inducing cellular level control. It has been observed that cells will respond to the substrate’s ordered microstructure by polarizing themselves to align in the direction of flow deposition.
Another application of orientation dynamics is in the development of solar cells. The efficiency of second-generation solar cells fabricated with conjugated polymers is limited by photoelectron transport within the polymer film. Inspired by electrorheological fluids, an external electric field is applied to the film to induce anisotropy in polymer crystallites, which is expected to enhance electron mobility.
The microstructure of polymeric and other complex materials also cause them to have interesting physical properties and respond to different flow conditions in unusual manners. In our laboratory, we are equipped with instruments that are able to characterize these materials such as shear rheometer, capillary break up extensional rheometer, and 2D extensional rheometer. Then, the response of these materials to different flow conditions can be visualized and analyzed in detail using high speed imaging devices at up to 2,000 frames per second.
Interfacial Dynamics and Rheology
There are numerous processes encountered in nature and industry where the deformation of fluid-fluid interfaces is of central importance. Examples from nature include deformation of the red blood cell in small capillaries, cell division and structure and composition of the tear film. Industrial applications include the processing of emulsions and foams, and the atomization of droplets in ink-jet printing. In our laboratory, fundamental research is in progress to understand the orientation and deformation of monolayers at the molecular level. These experiments employ state of the art optical methods such as polarization modulated dichroism, fluorescence microscopy, and Brewster angle microscopy to obtain in situ measurements of polymer films and small molecule amphiphile monolayers subject to flow. Langmuir troughs are used as the experimental platform so that the thermodynamic state of the monolayers can be systematically controlled. For the first time, well characterized, homogeneous surface flows have been developed, and real time measurements of molecular and microdomain orientation have been obtained. These microstructural experiments are complemented by measurements of the macroscopic, mechanical properties of the films.