Studies of Viscophoresis -- Drift in a Viscosity Gradient

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Non-technical Abstract:By applying a difference in pressure, voltage, or temperature to the ends of a tube filled with liquid, we can cause the fluid or small particles within it to move, and the mechanisms underlying the motion are well understood. It was recently discovered that when a small glass tube is filled with liquid and the viscosity is held at different values at either end, an electrical current begins to flow; the origin of that motion of electrical charges is not understood at present. This project is experimentally investigating the origins of that phenomenon by measuring the motion of salt ions and small fluorescent particles inside nanofluidic channels, where the physical dimensions and the properties of the liquid can be carefully controlled. The results shed new light onto the theory of the Brownian motion - the random motion of small particles caused by its collisions with the molecules of a liquid. Remarkably, more than a century after the publication of Einstein's famous theory of Brownian motion, the question of whether a particle drifts in a viscosity gradient, and if so, in which direction, is still debated. The conventional picture of the Brownian motion is a walk with steps taken in random directions and step sizes that are inversely proportional to the viscosity. When the viscosity varies in space, one must choose whether the size of each step in the random walk corresponds to the viscosity at the beginning of the step, the end of the step, or somewhere in between, and this seemingly insignificant choice has measurable consequences. In fact, different choices can be appropriate in different physical situations. Experiments can settle the debate and deepen our understanding of microscopic phenomena that are important in living cells, in industrial applications, and possibly beyond. The team is producing a scientific cartoon to explain the Brownian motion and its fundamental importance to a broad audience of non-specialists. Technical Abstract:This project is exploring the origins of a recently discovered nanofluidic transport phenomenon whereby small particles in solution drift due to a gradient in the viscosity of the solvent. The team has named this transport phenomenon "viscophoresis". Measurements of the transport of ions and fluorescent quantum dots in nanofluidic devices with well-defined dimensions establish the quantitative relationships between the drift velocity, the viscosity gradient, and other experimental parameters. The results of those experiments are needed to shed new light on the longstanding "Ito-Stratonovich" theoretical problem of noise-driven dynamics with multiplicative noise. Viscophoresis is significant because nanofluidic transport processes play a vital role in biology, in geology, and in micro- and nanofluidic technology. This project can lead to new insights or applications in those domains, where large viscosity gradients frequently arise. This project provides an excellent training opportunity for a graduate student, and an educational scientific cartoon is being created to reach a broad audience of non-specialists.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.