Search

Liquids are known to slip past non-wetting channel walls. The degree of slippage can be patterned locally through engineered variations in topography and/or chemistry. Electro-osmotic flow through a thin slit-like nanochannel with walls of sinusoidally varying slippage is studied through an asymptotic theory that uses the ratio of pattern amplitude to the average slip as a small parameter. The direction of patterning is perpendicular to the applied electric field. No restrictions are placed on the relative magnitudes of the channel height, wavelength of the pattern, the average slip length and the phase shift between the patterns on the walls. A closed-form analytical expression is provided for the effective slip length and tested against limits known from the literature. The results are also generalized for applicability to any unidirectional flow field that might originate from other driving forces such as pressure differential. The asymptotic results are compared with numerical simulations and are found to be in good mutual agreement even for moderate magnitudes of the small parameter in the asymptotic theory. [ABSTRACT FROM AUTHOR]

This work reports an experimental study of the thermal effect on the deposition of microparticles onto a solid surface in a microfluidic system, which allows a precise control of the solution temperature and enables the real-time monitoring of the deposition kinetics at the increased temperature. The static particle deposition rate (Sherwood number) has been measured over a range of temperatures between 20 and 70 °C. It is found that the Sherwood number is monotonically increased up to 265 %, with the solution temperature within the test range. A model including the Derjaguin-Landau-Verwey-Overbeek theory-based colloidal surface forces and gravity force is employed, taking into account temperature effects, to qualitatively interpret the experimental findings. The model shows that, by increasing the solution temperature, the attraction energy (van der Waals force) between the particles and the solid surface is increased while the repulsive energy (electric double layer force) is decreased. These findings demonstrate the importance of thermal effects in various thermally driven deposition processes, such as the fouling of bacteria and milk proteins in microscale milk pasteurization units. [ABSTRACT FROM AUTHOR]

In this paper, we show that large connected slip patches (hydrophobic patches) are a necessity to induce macroscopic slip effects of water flow in microchannels. For this purpose, the 2D fluid flow between a planar stationary surface with alternating stick and slip patches and a parallel planar surface moving with a constant relative velocity has been studied by computer simulations based on Navier-Stokes equations. A slip patch is defined as the slipping length in a 2D system or a slip area of the surface in a 3D system. The simulations reveal that the ratio (size of each slip patch)/(distance between the two parallel interfaces) has profound effect on the viscous stress on the moving surface when this ratio is around and above one. However, when the ratio is much below one, the effect of the slip patches are minor, even if the area fraction of slip patches are higher than 50 %. Obviously, the stick patches adjacent to the slip patches act as effective barriers, preventing the fluid velocity to increase near the surface with alternating stick and slip patches. The obtained results are scalable and applicable on all length scales, with an exception for narrow channels in the subnano regime, i.e. <1 nm where specific effects as the atomistic composition and the nanostructure of the wall as well as the interactions between the wall and the water molecules have an effect. [ABSTRACT FROM AUTHOR]

Nanoscale roughness in combination with surface chemistry modification can achieve large liquid slippage due to entrapment of air pockets in the surface asperities (superhydrophobic Cassie state). However, pressure increase may result in the collapse of the gaseous enclaves into the fully wet Wenzel state, and in the irreversible loss of the desirable superhydrophobic features. In order to explain the subtle effects of pressure, an extensive simulative campaign based on full-atoms molecular dynamics and aimed at reproducing a realistic superhydrophobic system comprising water, solid walls, and hydrophobic coating with octadecyltrichlorosilane is addressed in comparison with a continuum approach. The dramatic impact of pressure on slippage is twofold: on the one hand, it influences the shape of the liquid/vapor interface, thus indirectly affecting the flow. This is a purely geometrical effect well captured in principle by the continuum description. On the other hand, pressure controls the complex interplay between water and hydrophobic chains close to the triple line where it significantly alters the liquid structure, thus modifying apparent slippage. This is an intrinsically atomistic phenomenology that cannot be predicted at the continuum level. The combination of the two effects is found to explain the peculiar response of slippage to the applied pressure at the nanoscale, that may look at first sight in contradiction with most microscale literature results. [ABSTRACT FROM AUTHOR]

This paper presents analytical and numerical results for Poiseuille flow through microtubes patterned with superhydrophobic surfaces which consist of alternative ribs and grooves aligned longitudinally with the flow direction. The superhydrophobic surface prevents the flowing liquid from penetrating the grooves and the liquid-gas interface experiences deformation as a consequence of a pressure difference across the interface. Employing a domain perturbation technique, the effects of a small interface deformation on the effective slip behavior are analytically quantified. For large interface deformations, numerical studies are performed to predict the effects of interface protrusion on the effective slip behavior of the superhydrophobic microtube. Comparisons are made between the effective slip behavior for tube and channel flows patterned with superhydrophobic surfaces containing alternating longitudinal ribs and grooves. [ABSTRACT FROM AUTHOR]

This paper reports on simulations of electrokinetic transport in an electrolyte nanofilm in contact with a negatively charged substrate with discontinuous counter-charged patches. All-atom molecular dynamics simulations of an aqueous solution of chloride placed on a silica slab were conducted. The response of the system is investigated as it is subjected to different axial electric fields and as the surface electrostatics is modified by implementing different positive charge densities on the patches. Chloride and water density profiles reveal that the charged patches cause an enrichment of hydrated chloride ions near the surface. Therefore, the system with patches proved to be successful in trapping ions. Moreover, the number of chloride ions trapped on the surface increases as higher charge densities are imposed on the patches. It is observed that higher axial electric fields remove chloride ions from the surface; however, the amount removed decreases as higher charge densities are imposed on the patches. Velocity profiles show a stagnant water layer near the patches. The water and ion accumulation suggest the formation of a Stern layer with thickness of ca. 0.4 nm on the patches. Higher chloride density near the surface and increased surface-water affinity induced by the presence of charged patches result in smaller electroosmotic velocities. The simulation results compare well with velocities calculated using a continuum approach. This study shows that in a charged electrolyte solution in contact with a substrate, axial and normal ion distributions and consequently electroosmotic velocities can be controlled by modifying the electrostatics of the substrate. [ABSTRACT FROM AUTHOR]

The effect of rough-wall/fluid interaction on flow in nanochannels is investigated by NEMD. Hydrophobic and hydrophilic surfaces are studied for walls with nearly atomic-size rectangular protrusions and cavities. Our NEMD simulations reveal that the number of liquid atoms temporarily trapped in the cavities is affected by the strength of the potential energy inside the cavities. Regions of low potential energy are possible trapping locations. Fluid atom localization is also affected by the hydrophilicity/hydrophobicity of the surface. Potential energy is greater between two successive hydrophilic protrusions, compared to hydrophobic ones. Moreover, groove size and wall wettability are factors that control effective slip length. Surface roughness and wall wettability have to be taken into account in the design of nanofluidic devices. [ABSTRACT FROM AUTHOR]

In this article, we investigate the implications of electroosmosis with interfacial slip on electrohydrodynamic transport in microchannels having complex (yet symmetric) cross-sectional shapes, by employing a generic semi-analytical approach. We also devise an approximate technique of flow rate prediction under these conditions, using a combined consideration of electroosmotic slip (under thin electrical double layer limits) and Navier slip conditions (originating out of confinement-induced hydrophobic interactions) at the fluid-solid interface. We further assess the effectiveness of the approximate solutions in perspective of the exact solutions, as a parametric function of the relative thickness of the electrical double layer with respect to the channel hydraulic diameter. We illustrate the underlying consequences through examples of elliptic, polygonal, point star-shaped, and annular microchannel cross sections. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulation is a powerful tool to investigate the nanoscale fluid flow. In this article, we review the methods and the applications of MD simulation in liquid flows in nanochannels. For pressure-driven flows, we focus on the fundamental research and the rationality of the model hypotheses. For electrokinetic-driven flows and the thermal-driven flows, we concentrate on the principle of generating liquid motion. The slip boundary condition is one of the marked differences between the macro- and micro-scale flows and the nanoscale flows. In this article, we review the parameters controlling the degree of boundary slip and the new findings. MD simulation is based on the Newton's second law to simulate the particles' interactions and consists of several important processing methods, such as the thermal wall model, the cut-off radius, and the initial condition. Therefore, we also reviewed the recent improvement in these key methods to make the MD simulation more rational and efficient. Finally, we summarized the important discoveries in this research field and proposed some worthwhile future research directions. [ABSTRACT FROM AUTHOR]