Microfabricated fluidic devices have the potential to revolutionize chemistry, biology, and medicine, much as the integrated circuit did for computing, science and technology. Sophisticated devices have already been developed to perform tasks that would be vastly more expensive, more difficult, or even impossible with macro-scale techniques. If microfluidic flows can be driven in a self-contained, portable fashion, such devices could be taken out of the lab and into the field (or under the skin). Electrokinetic flows present many advantages towards microfluidic portability; however, fundamental issues have thus far precluded their use in practical systems.
This CAREER proposal describes a theoretical and experimental program with two central goals: 1) a fundamental understanding of electro-osmotic flow over liquid/liquid interfaces, and 2) the exploitation of these and related phenomena to develop truly portable microfludic manipulation systems. To achieve the first goal, the PI will develop and employ a microfluidic system that will enable, for the first time, direct measurements of electro-osmotic flow over liquid/liquid interfaces, while providing a control over the input 'variables' that is unprecedented (even impossible) in colloidal studies. As such, this will allow the first direct and stringent test of electrokinetic theories. To achieve the second goal, the PI will develop a low-voltage, high-pressure microfluidic pump that employs transverse induced-charge electrokinetic phenomena within a novel anisotropic porous bed.
Intellectual Merit: The microfluidic platform proposed will allow groundbreaking, fundamental studies in a field that is nearly two centuries old. Previous colloidal studies probed electrokinetic flows only indirectly, and allowed little or no control over surface charge density, geometry, or double-layer dynamics. The proposed system allows direct control over all of these quantities, and directly measures the consequent flows. A variety of physical regimes will thus be available for study: linear and nonlinear electrokinetics, transient double-layer effects, and surface conductivity. The direct application of the resulting new knowledge to microfluidic manipulation systems will significantly broaden our understanding of induced-charge electrokinetics, both in testing theories for asymmetric bodies and in developing statistical theories for concentrated collections. In all cases, the PI will emphasize the simplest, most intuitive systems to elucidate key phenomena.
Broader Impacts: The proposed electrokinetic pump may be immediately integrated into existing elastomeric microfluidic devices for rapid and broad impact. This will enable an entirely portable, robust, and versatile fluidic manipulation system and make possible hand-held hazard sensors and medical diagnostic tools, as well as implantable biomedical devices. The PI will continue his efforts to bridge the divide between the "application" and "fundamental" communities in microfluidics, and has designed this CAREER program to demonstrate the value of fundamental understanding in engineering solutions to real-world challenges, and the impact one can have by seriously considering real-world challenges in designing fundamental research. He will promote this view in his role as the "fundamentals expert" on the editorial board of the new American Institute of Physics journal Biomicrofluidics. He will continue to leverage existing, successful programs at UCSB (such as the California Alliance for Minority Participation) to integrate undergraduates and under-represented minorities into his research, and will include high-school students and teachers.
Education: The PI seeks to re-invigorate student interest in fluid and transport phenomena by using microfluidics as an exciting motivational framework, by emphasizing physically intuitive understanding, and by addressing the variety of disciplines and applications that depend on such phenomena. He will use his review article on microfluidic physical phenomena as the basis for a multidisciplinary special-topics course and as the foundation for a textbook. He will develop and web-publish a freshman seminar course in microfluidics for non-scientists/engineers to broaden the impact of the research, and to more generally cultivate an appreciation for the variety of interesting, exciting, surprising and beautiful phenomena that occur in microfluidics.
Electrokinetic phenomena involve the interaction of ions, fluids, electric fields and suspended particles. Examples include the electrophoretic migration of particles and molecules under applied fields, which was used in the original sequencing of the human genome; streaming potentials and currents, which can cause explosions in refineries, tankers, and gas pumps if not prevented; and electro-osmotic fluid flows, which can rapidly drive fluids through micro- and nanofluidic devices and membranes, and desalination through electrodialysis. Despite two centuries of study, new surprises and technologies continue to emerge. This research focused on fundamental and applied aspects of induced-charge, electrokinetic (ICEK) flows, in which an applied electric field induces an ionic double-layer around a polarizable surface, then forces that induced ionic cloud (and the fluid around it) into motion. These flows hold both applied and fundamental scientific appeal because they enable direct and falsifiable predictions to be compared directly against experimental measurements, and thus tests of the standard theories of ion transport and fluid mechanics. The research developed a high-throughput experimental system for quantitative ICEK studies, which probed thousands of experimental conditions to reveal significant discrepancies between theoretical predictions and experimental measurements. Results highlighted the role that surface chemistry plays in the ultimate ICEK flows, mechanisms by which ICEK flows can be enhanced using liquid metals, and also factors that suppress ICEK flows, including surface contamination and high conductivity along curved or discontinuous surfaces (e.g. nanoscale roughness, electrode edges, and colloidal particles or mobile surfaces). The results suggest ways to optimize fast ICEK flows, and design microfluidic systems that exploit ICEK phenomena. A second focus area involved bio-sensors, and sensors more generally. Even the most sophisticated and sensitive sensor can detect nothing unless and until the relevant molecules actually reach the sensor. The research clarified how much time is required for this to occur – the competing effects of molecular diffusion, convection with the flowing fluid, and molecular reaction each place fundamental limits on how quickly molecules can be detected. Physically intuitive scaling arguments were developed that enable quick but accurate estimates of transport time and (and thus sensor flux), without requiring detailed computational studies or advanced mathematics. Throughout the research, simple, paradigmatic systems were identified to most clearly elucidate the central phenomenon under investigation, and thus to develop and convey the physical intuition required for the rational design and engineering of more complex systems. Grant funds were additionally used to train two Ph.D. students – one currently working in Lawrence Livermore National Laboratory – and multiple undergraduate students towards STEM careers, and to develop and deliver short courses and workshops on electrokinetics and microfluidics to academic and industrial researchers alike. Research results were published in the peer-reviewed literature and through invited and contributed presentations at national and international conferences.
