Hossein P. Kavehpour, University of California-Los Angeles
This project is an experimental investigation to probe the evolution of the free surface and nano-structures in the vicinity of the moving contact line for polymeric fluids. Recent theoretical and experimental investigations have shown that the shape of the free surface may be markedly different for highly entangled polymer melts. A microscopic film, which is known as the precursor layer, exists at the front of the moving contact line for wetting fluids. The structure of this thin film has been studied theoretically, but previous experimental investigations were limited by the resolution of the measurement system (lateral or vertical) required to capture the complete scope of this feature. The free surface close to the moving contact line of highly-entangled polymer melts features another type of structure, called the "foot region." This structure arises from formation of unentangled polymers close to the moving contact line, due to the high shear stress. The existence of the microscopic precursor film is still a possibility for these types of fluids. Our preliminary experiments have shown such structures but additional experiments are necessary. The PI will measure the evolution of the profile of a spreading droplet near the moving contact line using a phase-shifted laser feedback interferometer (psLFI) and a total internal reflection fluorescence microscope (TIR-FM). The psLFI system has the vertical resolution of approximately 10nm and lateral resolution of 0.5im. This system will measure the free surface evolution at the macroscopic and microscopic levels. The TIR-FM system can detect fluorescence materials approximately 100nm from the substrate. This system, commonly used on single cell molecule dynamics, has not been used to study the contact line phenomena. The range and resolution of the TIR-FM makes it an ideal tool for such experiments. The experimental finding will be compared with the available theoretical predictions. The intellectual merit of this investigation rests on the fact that most of the previous experiments done on this problem did not have appropriate temporal and spatial resolution to fully quantify this phenomena. Using psLFI and TIR-FM will provide resolution and dynamic range required to better investigate the moving contact line of a variety of complex fluids. These experiments will reveal three dimensional structure of precursor film for the first. The broad impact will benefit numerous industrial fields, from coating and paint industries to. In addition, this grant will further the training of a Ph.D. graduate student and an undergraduate researcher and therefore integrate research and education.
