Effect of boundary slip and surface charge on the pressure-driven flow

Electromagnetohydrodynamic (EMHD) flow of Jeffrey fluid through a rough circular microchannel with surface charge-dependent slip

This research examines the electromagnetohydrodynamic (EMHD) flow of Jeffrey fluid in a rough circular microchannel while considering the effect of surface charge on slip. The channel wall corrugations are described as periodic sinusoidal waves with small amplitudes. The perturbation method is employed to derive solutions for velocity and volumetric flow rate, and a combination of three-dimensional (3D) and two-dimensional (2D) graphical representations is utilized to effectively illustrate the impacts of relevant parameters on them. The significance of the Reynolds numberR e $Re$ in investigations of EMHD flow is particularly emphasized. Furthermore, the effect of wall roughness ε $\varepsilon $ and wave number k $k$ on velocity and the influence of wall roughness ε $\varepsilon $ and surface charge densityσ s ${\sigma }_s$ on volumetric flow rate are primarily focused on, respectively, at various Reynolds numbers. The results suggest that increasing the wall roughness leads to a reduction in velocity at low Reynolds numbers (R e = 1 $Re = 1$ ) and an increment at high Reynolds numbers (R e = 10 $Re = 10$ ). For any Reynolds number, a roughness with an odd multiple of wave number (k = 6 , 10 $k = 6,10$ ) will result in a more stable velocity profile compared to one with an even multiple of wave number (k = 4 , 8 $k = 4,8$ ). Decreasing the relaxation timeλ ¯ 1 ${\bar{\lambda }}_1$ while increasing the retardation timeλ ¯ 2 ${\bar{\lambda }}_2$ and Hartmann numberH a $Ha$ can diminish the impact of wall roughness ε $\varepsilon $ and surface charge densityσ s ${\sigma }_s$ on volumetric flow rate, independent of the Reynolds number. Interestingly, in the existence of wall roughness, further consideration of the effect of surface charge on slip leads to a 15% drop in volumetric flow rate atR e = 1 $Re = 1$ and a 32% slippage atR e = 10 $Re = 10$ . However, in the condition where the effect of surface charge on slip is considered, further examination of the presence of wall roughness only results in a 1.4% decline in volumetric flow rate atR e = 1 $Re = 1$ and a 1.6% rise atR e = 10 $Re = 10$ . These findings are crucial for optimizing the EMHD flow models in microchannels.

Thermal and Flow Analysis of Fully Developed Electroosmotic Flow in Parallel-Plate Micro- and Nanochannels with Surface Charge-Dependent Slip

This study analytically investigates the coupled effects of surface charge and boundary slip on the fully developed electroosmotic flow and thermal transfer in parallel plate micro and nanochannels under the high zeta potential. The electric potential, velocity, temperature, flow rate, and Nusselt number are obtained analytically. The main results are that the velocity of bulk flow is significantly reduced in the presence of the surface charge-dependent slip. Moreover, the maximum velocity at ζ = -125 mV is approximately twice as large as that at ζ = -25 mV. The velocity and dimensionless temperature increase as the zeta potential increases. The dimensionless temperature of the surface charge-dependent slip flow is larger than that of the surface charge-independent slip flow. For the surface charge-dependent slip flow, the maximum temperature at ζ = -125 mV is approximately four times larger than that at ζ = -25 mV. The Nusselt number decreases with Joule heating and increases with a positive heat transfer coefficient. The Nusselt number decreases as the electric field and the magnitude of the zeta potential increase. In the surface charge-dependent slip flows, the Nusselt number is smaller than that in the surface charge-independent slip flows.

Influence of Surface Texture on the Variation of Electrokinetic Streaming Potentials

The electrokinetic streaming potential (V_s) obtained through electrolyte flow in a microchannel is shown to be related to the underlying surface pattern. Pillar, mesh, and groove patterns were studied for comparing the relative magnitudes of the V_s with air-/liquid-filled surfaces. A record value of the related figure of merit, in terms of the developed V_s per-unit applied pressure, of ∼0.127 mV/Pa, was observed in a mesh texture liquid-filled surface (LFS) impregnated with an electrolyte-immiscible oil. The study indicated that increasing the solid fraction of the pattern surface decreases the effective slip length while enhancing the overall channel ζ potential. Consequently, maximizing the obtained V_s implies a balancing of the slip with the surface potential, with plausibly more significance of the latter. The work has implications for higher-efficiency electrical voltage sources.

Transport behavior of pressure-driven electrolyte solution through a surface-charged nanochannel

The transport behavior of a pressure-driven electrolyte solution through a surface-charged nanochannel is investigated using molecular dynamics (MD) simulations. Similar to pure water, the relationship between the applied pressure (P) and the average transport velocity ([Formula: see text]) of the electrolyte solution is roughly linear, which matches with the theoretical solution very well. The friction coefficient λ is used to describe the transport behavior (a higher λ leads to a lower [Formula: see text]), which scales with the slope of the P- [Formula: see text] relationship and increases with the increases of both the charge density of the channel wall σ and the electrolyte concentration n. The physical mechanism is found as follows: the solid-liquid interaction energy between the channel wall and the liquid inside the channel decreases with both σ and n (being more negative), which makes it more difficult for the liquid boundary layer to slide against the channel wall, leading to a higher λ. In addition, the increase of σ also causes a significant decrease of the liquid-liquid interaction energy but the opposite effect is found with the increase of n. However, λ increases with the increase of both σ and n, suggesting that the relationship between λ and the liquid-liquid interaction energy is more complicated for an electrolyte solution, different from the corresponding result of pure water.

Slip Effects on Ionic Current of Viscoelectric Electroviscous Flows through Different Length Nanofluidic Channels

The pressure driven slip flow of an electrolyte solution is studied through different nanofluidic channel lengths at varying salt concentrations. The viscous-thickening due to the electrostatic interactions within the electric double layer and the reverse ionic transport due to the streaming potential are developed. The influence of the Navier slip boundary condition is described under both electroviscous and viscoelectric effects with a surface charge regulation (CR) model while the observed behavior is compared and validated with molecular dynamic (MD) calculations from multiple studies. Results show that electroviscous and viscoelectric effects decrease transport. Earlier studies at the no slip boundary presented an increase of ionic current by increasing salt concentration and decreasing channel length. In contrast, our study found that the ionic current occurred almost independent of both salt concentration and channel length, except for very short channels and very low salt concentrations, when electroviscous and viscoelectric effects were considered. In the case of the constant slip length condition, ionic conduction was enhanced, but velocity slip developing on surfaces showed significant variation based on the salt concentration and channel length. This is due to the natural CR behavior enhancing the surface charge and consequential near surface electrohydrodynamics as a result of increase in salt concentration and/or decrease of channel length. Considering that the electroviscous effect alone creates up to 70% lower velocity slips than Poiseuille flow predictions, while further including the viscoelectric effect, results in an almost no-slip condition at high salt concentrations and/or short channels. As a result, the ionic current of a viscoelectric electroviscous slip flow is found to be equal to 1/3 of an electroviscous slip flow and to decrease with a decrease in the channel length.

Effects of surface charge and boundary slip on time-periodic pressure-driven flow and electrokinetic energy conversion in a nanotube

Time-periodic pressure-driven slip flow and electrokinetic energy conversion efficiency in a nanotube are studied analytically. The slip length depends on the surface charge density. Electric potential, velocity and streaming electric field are obtained analytically under the Debye-Hückel approximation. The electrokinetic energy conversion efficiency is computed using these results. The effects of surface charge-dependent slip and electroviscous effect on velocity and electrokinetic energy conversion efficiency are discussed. The main results show that the velocity amplitude and the electrokinetic energy conversion efficiency of the surface charge-dependent slip flow are reduced compared with those of the surface charge-independent slip flow.

The coupling of surface charge and boundary slip at the solid-liquid interface and their combined effect on fluid drag: A review

Fluid drag of micro/nano fluidic systems has inspired wide scientific interest. Surface charge and boundary slip at the solid-liquid interface are believed to affect fluid drag. This review summarizes the recent studies on the coupling of surface charge and slip, and their combined effect on fluid drag at micro/nano scale. The effect of pH on surface charge of borosilicate glass and silica surfaces in deionized (DI) water and saline solution is discussed using a method based on colloidal probe atomic force microscopy (AFM). The boundary slip of various oil-solid interfaces are discussed for samples with different degrees of oleophobicity prepared by nanoparticle-binder system. By changing the pH of solution or applying an electric field, effect of surface charge on slip of a smooth hydrophobic octadecyltrichlorosilane (OTS) in DI water and saline solution is studied. A theoretical model incorporating the coupling relationship between surface charge and slip is used to discuss the combined effect of surface charge-induced electric double layer (EDL) and slip on fluid drag of pressure-driven flow in a one-dimensional parallel-plates microchannel. A theoretical method is used to reduce the fluid drag. The studies show that the increasing magnitude of surface charge density leads to a decrease in slip length. The surface charge results in a larger fluid drag, and the coupling of surface charge and slip can further increase the fluid drag. Surface charge-induced EDLs with asymmetric zeta potentials can effectively reduce the fluid drag.

The study of surface wetting, nanobubbles and boundary slip with an applied voltage: A review

The drag of fluid flow at the solid-liquid interface in the micro/nanoscale is an important issue in micro/nanofluidic systems. Drag depends on the surface wetting, nanobubbles, surface charge and boundary slip. Some researchers have focused on the relationship between these interface properties. In this review, the influence of an applied voltage on the surface wettability, nanobubbles, surface charge density and slip length are discussed. The contact angle (CA) and contact angle hysteresis (CAH) of a droplet of deionized (DI) water on a hydrophobic polystyrene (PS) surface were measured with applied direct current (DC) and alternating current (AC) voltages. The nanobubbles in DI water and three kinds of saline solution on a PS surface were imaged when a voltage was applied. The influence of the surface charge density on the nanobubbles was analyzed. Then the slip length and the electrostatic force on the probe were measured on an octadecyltrichlorosilane (OTS) surface with applied voltage. The influence of the surface charge on the boundary slip and drag of fluid flow has been discussed. Finally, the influence of the applied voltage on the surface wetting, nanobubbles, surface charge, boundary slip and the drag of liquid flow are summarized. With a smaller surface charge density which could be achieved by applying a voltage on the surface, larger and fewer nanobubbles, a larger slip length and a smaller drag of liquid flow could be found.