Probing start-up electroosmotic forces and flows in a microfluidic channel using laser tweezer force spectroscopy
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RESEARCH PAPER
Probing start‑up electroosmotic forces and flows in a microfluidic channel using laser tweezer force spectroscopy A. Raudsepp1 · S. B. Hall1 · M. A. K. Williams1,2 Received: 16 January 2020 / Accepted: 20 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Transient displacements of optically trapped particles in [NaCl] = 0.1 mM solutions produced by electroosmotic and electrophoretic forces at electric field start-up were profiled wall-to-wall through 50 μm in a commercial microfluidic channel with a spatial resolution of 1 μm and temporal resolution of 200 kHz. Data were inverted to compute the force on the particles and fitted to a first-principles finite element methods model to compute the flow profile, and zeta potential of the walls and particles. This analysis suggested that (1) electroosmotic flow in the channel was accompanied by a pressure gradient, producing backflow, and which was attributed to bubbles within the channel and that (2) while the zeta potential of the wall was broadly consistent with that expected, the zeta potentials across the nine particles examined was higher than might be expected, which were attributed to differences in surface conditions of the particular particles used. Keywords Optical tweezers · Microfluidics · Electroosmotic flow · Finite element methods
1 Introduction Since their advent in the 1990’s, microfluidic devices have become an increasingly popular platform for a wide range of scientific applications (Nge et al. 2013). Fluid transport in these devices can be an important design consideration. More commonly, fluid is moved through the channels of the device using a pressure gradient. While this approach can be simple, it has an important limitation: the flow profile across the channel is generally Poiseuille-like which results in Taylor dispersion (Aris 1956; Kirby 2010). Less commonly, an electric field is used to steer fluid in the device. Flow produced by an electric field in this way is called electroosmotic flow (EOF). By contrast to pressure-driven flow, EOF can be plug-like and does not suffer from Taylor * A. Raudsepp [email protected] S. B. Hall [email protected] M. A. K. Williams [email protected] 1
School of Fundamental Sciences, Massey University, Palmerston North 4442, New Zealand
MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6140, New Zealand
2
dispersion, which can be advantageous if this dispersion is to be avoided (Kirby 2010). EOF is due to an interaction of the applied electric field with charged counter-ions near the channel’s wall. In a fluidfilled channel, charges embedded in the wall lead to a separation of charge in solution and the formation of a diffuse layer of oppositely charged counter-ions near the wall’s surface. An electric field produces an electrophoretic force on these counter-ions, inducing movement. Momentum associated with this movement diffuses out from this layer into the fluid, resulting in plug-like flow at long times. T
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