The research objective of this Faculty Early Development (CAREER) program is to harness interfacial phenomena to achieve external, reversible, and local control of wetting and adhesion properties at the nanoscale. Strategies to create surface device elements by modulating a surface spatially (patterning) or dynamically (external stimulus) are developed and explored in fundamental experiments. To achieve the goals of this program, the surface force apparatus (SFA) will be employed as a testbed in concert with patterning, surface chemistry, and external fields. Specific aims of the proposed work are to (1) characterize the formation and stability of liquid bridges on surfaces patterned with stripes of alternating wetting properties in technologically relevant conditions and (2) create surface films that respond to electrical stimuli by changing conformation and altering wettability. The CAREER educational plan is designed to foster a true passion for science in students by giving them opportunities to actively produce scientific material. In the coursework developed, this takes the form of the traditional "see one, do one, teach one" framework, while exploiting the power and omnipresence of Wikipedia (www.wikipedia.org) to leverage students' efforts. In the lab, undergraduates are encouraged to present their findings within the group and externally. Finally, in cooperation with the National Federation of the Blind (NFB), workshops were created in the lab for visually impaired students for the NFB Youth Slam.
The proposed research promises to have significant scientific and technological impact. Microfluidic devices have the potential to revolutionalize biological and chemical analysis due to fast reaction times, small volumes, and parallel operations. In addition to microfluidics applications, the area of optofluidics is emerging as a combination of MEMS, microfluidics and optics. To enable optofluidic devices to reach their full technological potential, however, the creation and implementation of new means of control and actuation that can operate at the nanoscale in confined fluid environments are needed. Key areas that must be addressed to control fluid in nanochannels are the actuation and control of flow, the means to manipulate the local refractive index, and the introduction of mechanical components. This program will test new strategies based on manipulating surface interactions to both control the optical properties and actuate mechanical components at the nanoscale. The proposed work will address fundamental issues particularly pertinent in the design of specific devices, but anticipated to affect this growing field at large.
We live in a time of unprecedented opportunity to create new technologies that exploit assemblies and devices on the micro- and nanoscale. At these length scales, surface to volume ratios are large and the success of these endeavors relies over controlling and understanding surface and interfacial properties. We created powerful approaches to control any micro- and nanoscale systems by exploiting our ability to tailor the molecular structures, surface topographies, and local fields on bounding surfaces and fluid interfaces. The contact line where a liquid, solid, and vapor meet plays a decisive role in interfacial properties. Small changes at this length scale can result in dramatic reconfiguration of fluid interfaces and provide new avenues to actuate or assemble devices. Our efforts over that length scale have led to breakthrough in two areas. 1) Responsive or "smart" surfaces are designed to respond to an external stimuli such as a change in pH, voltage, light, or solvent conditions. Such materials have potential applications for separation, drug delivery, and as means to direct fluid flow in microfluidic devices. In this area we demonstrated that a voltage-induced molecular conformational on a surface can be employed to control drop motion. We also showed how adsorption of nanoparticles at fluid interfaces can be controlled by modulating electrostatic interactions. 2) Flip chip assembly is an important industry that relies on the formation of capillary bridges between solid substrates. By studying the effect of pad shape, orientation, and wetting properties we helped understand the mechanisms at play and suggest new avenues for design improvements. Our work in this area also has implication in cohesion of granular materials and bio-inspired adhesion. Important outreach activities included the development of a tactile short course on interfacial science of blind or visually impaired high school students through the National Federation of the Blind 2008 Youth Slam.
