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Curved channels are ubiquitous in microfluidic systems. The pressure-driven electrokinetic flow and energy conversion in a curved microtube are investigated analytically by using a perturbation analysis method under the assumptions of the small curvature ratio and the Reynolds number. The results indicate that the curvature of the microtube leads to a skewed pattern in the distribution of the electrical double layer (EDL) potential. The EDL potential at the outer side of the bend is larger than that at the inner side of the bend. The curvature shows an inhibitory effect on the magnitude of the streaming potential field induced by the pressure-driven flow. Since the spanwise pressure gradient is dominant over the inertial force, the resulting axial velocity profile is skewed into the inner region of the curved channel. Furthermore, the flow rate in a curved microtube could be larger than that in a straight one with the same pressure gradient and shape of cross section. The asymptotic solutions of the axial velocity and flow rate in the absence of the electrokinetic effect are in agreement with the classical results for low Reynolds number flows. Remarkably, the curved geometry could be beneficial to improving the electrokinetic energy conversion (EKEC) efficiency. [ABSTRACT FROM AUTHOR]

This study investigates the electromagnetohydrodynamic (EMHD) flow of fractional viscoelastic fluids through a microchannel under the Navier slip boundary condition. The flow is driven by the pressure gradient and electromagnetic force where the electric field is applied horizontally, and the magnetic field is vertically (upward or downward). When the electric field direction is consistent with the pressure gradient direction, the changes of the steady flow rate and velocity with the Hartmann number Ha are irrelevant to the direction of the magnetic field (upward or downward). The steady flow rate decreases monotonically to zero with the increase in Ha. In contrast, when the direction of the electric field differs from the pressure gradient direction, the flow behavior depends on the direction of the magnetic field, i.e., symmetry breaking occurs. Specifically, when the magnetic field is vertically upward, the steady flow rate increases first and then decreases with Ha. When the magnetic field is reversed, the steady flow rate first reduces to zero as Ha increases from zero. As Ha continues to increase, the steady flow rate (velocity) increases in the opposite direction and then decreases, and finally drops to zero for larger Ha. The increase in the fractional calculus parameter α or Deborah number De makes it take longer for the flow rate (velocity) to reach the steady state. In addition, the increase in the strength of the magnetic field or electric field, or in the pressure gradient tends to accelerate the slip velocity at the walls. On the other hand, the increase in the thickness of the electric double-layer tends to reduce it. [ABSTRACT FROM AUTHOR]

Energy conversion in micro/nano-systems is a subject of current research, among which the electrokinetic energy conversion has attracted extensive attention. However, there exist two different definitions on the electrokinetic energy conversion efficiency in literature. A few researchers defined the efficiency using the pure pressure-driven flow rate, while other groups defined the efficiency based on the flow rate with the inclusion of the effect of the streaming potential field. In this work, both definitions are investigated for different fluid types under the periodic electrokinetic flow condition. For Newtonian fluids, the two definitions give similar results. However, for viscoelastic fluids, these two definitions lead to significant difference. The efficiency defined by the pure pressure-driven flow rate even exceeds 100% in a certain range of the parameters. The result shows that in the case of viscoelastic flow, it is incorrect to define the energy conversion efficiency by pure pressure-driven flow rate. At the same time, the reason for this problem is clarified through comprehensive analysis. [ABSTRACT FROM AUTHOR]