The research objective of this award is to elucidate the mechanisms of viscous energy dissipation on superhydrophobic surfaces and to enable robust operation of micro and nanoelectromechanical systems (MEMS and NEMS) in water through surface engineering. MEMS and NEMS are miniaturized mechanical devices, which are fabricated using semiconductor processes and are suitable for numerous technological applications, including ultrasensitive bio-sensing. The research approach is to shrink the solid-water interface area of MEMS and NEMS by engineering these devices with superhydrophobic surfaces. The research will combine theoretical and numerical work with experiments. Superhydrophobic MEMS and NEMS will be fabricated based on the numerical and theoretical surface designs; the devices will be tested in water to assess the designs. The main device parameter to be optimized is the quality (Q) factor, which quantifies the viscous dissipation in water. The deliverables are scalable superhydrophobic materials and coatings for nanoscale device applications, high-performance MEMS and NEMS devices in water and a fundamental physical understanding of solid-liquid-gas interfaces under high-frequency oscillations.
The research will provide an enabling technology for MEMS and NEMS devices in water. Operation of MEMS and NEMS in water-based biochemical solutions is crucial for future mass and force sensing applications. MEMS and NEMS sensor elements can provide powerful new approaches to bio-threat detection, drug screening, and medical diagnostics in liquids. In scanning probe microscopy (SPM), force sensitivity of the microcantilever probe can be improved in liquids by the results of this research. Graduate and undergraduate students will be trained in a wide cross-section of engineering and physics including nanoscale surface engineering, nanometrology, and fluid dynamics through classroom instruction and participation in research. Outreach activities co-sponsored by Boston University will be used as a platform to engage K-12 students and disseminate the results of the research broadly.
Flow of a viscous liquid along a solid surface experiences drag. Drag puts a limit on the speed of marine vessels; it determines the cost of pumping liquid in a pipe; it reduces the efficiency of thermofluid systems — this list goes on. This project studied fundamental interactions between solid surfaces and water in order to reduce drag. In particular, the focus of the project was on micro and nanoscale mechanical resonators. These devices vibrate at their resonance frequencies just like guitar strings or tuning forks, and have a multitude of applications in technology. When immersed in water, however, the above-mentioned drag problem appears, and the intensity and the coherence of the vibrations of the resonators are reduced. Here, we developed approaches for engineering the surfaces of these devices so that they are superhydrophobic. Superhydrophobic surfaces strongly repel water. Using superhydrophobic surfaces, we demonstrated that the friction force decreases significantly for resonators. Our theoretical investigations suggested that a thin stable layer of air was completed under the water layer, including over the solid portions, giving rise to the observed reduced drag. The result is expected enable a number of devices, including biomedical sensors, making them efficient in water. In subsequent work, we developed a technique for measuring the pressure distribution in a microfluidic channel with deformable walls. The physical principle underlying this measurement is deceptively simple: the flow exerts pressure upon the confining walls of the channel; the response of the walls, in turn, provides information about the flow itself. There is, however, an inherent difficulty in formulating a mathematical description of such a system: the channel diameter is a function of the flow; conversely, the flow is a function of the channel height. To solve such a set of coupled equations describing interactions between deformable objects and flows, one needs to obtain the underlying constitutive relations of fluid-structure interactions. In this work, we showed how to obtain these constitutive relations in a deformable micro-channel. For a proof-of-principle demonstration, we fabricated a variety of microchannels with compliant walls and determined the pressure distributions in them accurately. This method can be applied universally as an accurate probe of a variety of flows with micron, and even possibly sub-micron, length scales. New techniques are always in need for analyzing biological flows — e.g., blood flow in microvessels where flow-structure interactions are critical, or flow in interstitium (the space between cells in a tissue) which determines transport into and out of cells. Motivated by these needs, we applied his method to biological flow measurements. The most significant aspect of this work was that it clearly showed a path toward determining the hydraulic properties of cell aggregates and monolayers that are cultured within micropatterned gels. This work and the developed technique may make an impact on the field of tissue engineering. The research has trained a total of three graduate students (two PhD students and one MS student). All students got training in fluid structure interactions at the micro and nanometer length scales; they also gained important technical skills, including micro/nanofabrication, surface engineering and sensitive measurement techniques. The research also trained a number of undergraduate students and high-school students. These students worked alongside graduate students and gained some technical skills (e.g., microfabrication, optical measurement approaches, numerical analysis). They also learned a great deal of the fundamental aspects of the research.
