Streaming potential and surface charge density of microporous membranes with pore diameter in the range of thickness

Characterization of molecule clustering and liquid transport at nearly ideal solid surfaces

Vapor adsorption, mobility, two-dimensional (monolayer formation) and three-dimensional (multilayer formation) clustering is evaluated. Two-dimensional vapor diffusion is compared to results obtained from molecular kinetic (MK) model fits. Three-dimensional clustering results in condensation of multimolecular vapor layers to thin films. Thin films are characte-rized by line tension and liquid spreading by hydrodynamic (HD) models. Although it is experimentally shown that steady-state wetting ranges are intersected by a chaotic slip-stick range, MK and HD models are combined to molecular hydrodynamic (MH) models with the aim to cover this slip-stick range. The results of MK, HD and MH model fits are, however rather poor (unphysical results). Thin film (α-phase) models are compared to thick film (β-phase) models. In order to improve model designs, established phenomenological relation-ships known from irreversible thermodynamics are presented. Forced wetting, expressed as generalized fluxes can be made dependent on multiple generalized conjugate forces which enables identification of dominant interactions to be introduced in future improved transport models.

Measuring Electric Charge and Molecular Coverage on Electrode Surface from Transient Induced Molecular Electronic Signal (TIMES)

Charge density and molecular coverage on the surface of electrode play major roles in the science and technology of surface chemistry and biochemical sensing. However, there has been no easy and direct method to characterize these quantities. By extending the method of Transient Induced Molecular Electronic Signal (TIMES) which we have used to measure molecular interactions, we are able to quantify the amount of charge in the double layers at the solution/electrode interface for different buffer strengths, buffer types, and pH values. Most uniquely, such capabilities can be applied to study surface coverage of immobilized molecules. As an example, we have measured the surface coverage for thiol-modified single-strand deoxyribonucleic acid (ssDNA) as anchored probe and 6-Mercapto-1-hexanol (MCH) as blocking agent on the platinum surface. Through these experiments, we demonstrate that TIMES offers a simple and accurate method to quantify surface charge and coverage of molecules on a metal surface, as an enabling tool for studies of surface properties and surface functionalization for biochemical sensing and reactions.

The effect of surface charge on the boundary slip of various oleophilic/phobic surfaces immersed in liquids

The reduction of fluid drag is an important issue in many fluid flow applications at the micro/nanoscale. Boundary slip is believed to affect fluid drag. Slip length has been measured on various surfaces with different degrees of hydrophobicity and oleophobicity immersed in various liquids of scientific interest. Surface charge has been found to affect slip length in water and electrolytes. However, there are no studies on the effect of surface charge on slip at solid-oil interfaces. This study focuses on the effect of surface charge on the boundary slip of superoleophilic, oleophilic, oleophobic, and superoleophobic surfaces immersed in deionized (DI) water and hexadecane and ethylene glycol, based on atomic force microscopy (AFM). The surface charge was changed by applying a positive electric field to the solid-liquid interface, and by using liquids with different pH values. The results show that slip length increases with an increase in applied positive electric field voltage. Slip length also increases with a decrease in the pH of the solutions. The change in slip length is dependent on the absolute value of the surface charge, and a larger surface charge density results in a smaller slip length. In addition, the surface charge density at different solid-liquid interfaces is related to the dielectric properties of the surface. The underlying mechanisms are analyzed.

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.

Quantification of surface charge density and its effect on boundary slip

Reduction of fluid drag is important in the micro-/nanofluidic systems. Surface charge and boundary slip can affect the fluid drag, and surface charge is also believed to affect boundary slip. The quantification of surface charge and boundary slip at a solid-liquid interface has been widely studied, but there is a lack of understanding of the effect of surface charge on boundary slip. In this paper, the surface charge density of borosilicate glass and octadecyltrichlorosilane (OTS) surfaces immersed in saline solutions with two ionic concentrations and deionized (DI) water with different pH values and electric field values is quantified by fitting experimental atomic force microscopy (AFM) electrostatic force data using a theoretical model relating the surface charge density and electrostatic force. Results show that pH and electric field can affect the surface charge density of glass and OTS surfaces immersed in saline solutions and DI water. The mechanisms of the effect of pH and electric field on the surface charge density are discussed. The slip length of the OTS surface immersed in saline solutions with two ionic concentrations and DI water with different pH values and electric field values is measured, and their effects on the slip length are analyzed from the point of surface charge. Results show that a larger absolute value of surface charge density leads to a smaller slip length for the OTS surface.

Electrokinetic and surface chemical characterizations of an irradiated microfiltration polysulfone membrane: Comparison of two irradiation doses

The effect of ionizing radiation on the surface and electrokinetic characteristic parameters for a porous membrane of pore size 0.2 mum is determined and correlated with the irradiation dose (10 and 80 J/kg). Changes in NaCl permeability and membrane system electrical resistance determined from diffusion and impedance spectroscopy measurements are consistent with the increase of membrane pore radii/porosity, in agreement with SEM micrographs and reported results. Low irradiation dose seems to clean the membrane surface of impurities, according to XPS results, but the increase of irradiation doses could affect surface roughness. Due to the relatively high pore radius, ion transport numbers are practically independent of radiation and dose, but irradiation slightly modifies the membrane solution interface by increasing its weakly electronegative character, which could be of interest in the ultrafiltration of proteins or macromolecules.

Streaming potential across cation-exchange membranes in methanol–water electrolyte solutions

Streaming potential measurements across charged membranes separating two equal solutions have been carried out. Two cation-exchange membranes with different cross-linked and swelling properties (Ionics and Nafion membranes) and methanol-water electrolyte solutions of KCl have been used in the experiments. The obtained results show that the streaming potential is higher for the Ionics membrane and that the values depend on the methanol content of the solutions. A different behavior is found in the dependence of the streaming potential on the methanol percentage for each membrane. The study of the relaxation times in the decay of electrokinetic steady states of streaming potential has been carried out from the time dependence of the streaming potential when the pressure difference through the membrane is suppressed. The results show the existence of two different parts or partial relaxations, mechanical and electric. A different behavior of the mechanical relaxation time with the methanol percentage has been found for the two membranes, but any significant difference between their electric relaxation times. These differences have been explained in terms of the different degree of swelling of the membranes used.