Gradients of voltage, pressure, temperature, and salinity can transport objects in micro- and nanofluidic systems by well known mechanisms. Relatively little experimental work has previously been done to explore the behavior of particles in a viscosity gradient. This thesis presents observation, analysis, theory, and simulation of a new nanofluidic transport phenomenon whereby a gradient in liquid viscosity causes an ionic current to flow inside a glass nanofluidic channel. We studied ionic transport inside nanofluidic devices in which we set up a controlled viscosity gradient by pumping fluids of known viscosity past either end of a channel with no applied voltage, pressure, or salinity gradient. We measured currents on the order of 10 to 100 pA flowing in the direction of lower viscosity through the 200 μm-long and 150 μm-wide channels using fluids with viscosities that varied from 1 to 5 mPas. The nanofluidic devices enabled us to thoroughly characterize the current's dependence on experimental parameters like the viscosities of the liquids, the length of the channel, the surface charge density, and the bulk salinity. The currents increased linearly with the gradient of the inverse viscosity and the channel's surface charge density, but were insensitive to the bulk salinity. We propose a simple model of these viscosity-driven currents in which mobile counterions screening the channels' surface charge drift with a speed equal to the gradient in their diffusivities. This model describes our data well and explains the microscopic origin of the effect. Drift in a viscosity gradient is a consequence of multiplicative (state-dependent) noise, which refers to the dependence of a particle's thermal fluctuations on its position. The mathematical Itô-Stratonovich dilemma arises because one must choose whether the size of each stochastic step corresponds to the viscosity at the beginning of the step (Itô convention), the middle of the step (Stratonovich convention), the end of the step (isothermal convention), or somewhere in between. This seemingly insignificant choice has measurable consequences, as only the isothermal convention explains the existence and direction of the currents we measured. We present simulations which illuminate this surprising fact and show how the drift of ions arises from particles taking larger average steps when they move in the direction of decreasing viscosity.
Wiener, Benjamin,
"Electrokinetic Current Driven by a Viscosity Gradient"
(2019).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.26300/nkwr-dp72
