Electrokinetic flow imaging in well-designed microfluidic channels to analyze hydrodynamic slip behavior

Electrokinetic flow imaging in well-designed microfluidic channels to analyze hydrodynamic slip behavior
hydrodynamic slip; slip length; hydrophobicity; shear rate; particle streak velocimetry; fluorescent microscope
Issue Date
10th ELKIN 2012
VOL 10, 124-124
Hydrodynamic fluid slippage at the hydrophobic stationary interface can be interpreted in terms of the continuity of shear stress and the interfacial friction [Navier, Mem. Acad. Sci. Inst. Fr., 1823; Joly et al., J. Chem. Phys., 2006]. The slip length is the local equivalent distance below the solid surface at which the no-slip boundary condition would be satisfied if the flow fields were extended linearly outside of the physical domain [Lauga and Stone, J. Fluid Mech., 2003; Lim and Chun, Phys. Fluids, 2011]. Particle streak imaging by fluorescent microscope (shown in Fig. 1) can be applied to obtain electrokinetic velocity profile and fluid slip, as reported earlier [Chun and Lee, Colloids Surf. A, 2005]. Moving submicron-sized fluorescent polystyrene latex beads results in image streaks, where the dispersion concentration is sufficiently dilute underlying the condition of simple fluid. The local velocity was determined in terms of a ratio of the real distance to the number of pixels. For the purpose of fast and accurate measurements of slip lengths at different shear rates and channel widths, we fabricated a branched multi-channel type PDMS (polydimethylsiloxane) microfluidic chip that contains two main streams. The test fluid that enters the first stream undergoes a series of step reductions of shear rate, while the fluid that enters the second stream experiences a series of different channel widths, as displayed in Fig. 1. On-chip switching valve is also designed for the test fluid to enter only one specified stream. We observed that, in Fig. 2, the slip length is nearly constant in the range of shear rate less than about 100 s–1, but increases with the shear rate above 100 s–1.
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