Effective boundary slip and wetting characteristics of water on substrates with effects of surface morphology

ABSTRACT

In the present study, molecular dynamics (MD) simulation was used to investigate the relationship between wetting behaviour and slip length on patterned substrates. We adopted two solid surfaces of Si(100) and graphite due to similarities in their intrinsic contact angle. Contact angle and apparent slip length were obtained using discrete simulations with the same thermodynamic states. In the present study, a number of questions regarding surface roughness and the problem of contact angle (θ) and slip length (L_s) are discussed. These questions include the relationship between θ and surface roughness, the characteristics used to describe the difference between static and dynamic fluid fields and the reason for a lack of multilayer sticking observed in the current cases. Our results indicate that the quasi-universal θ ? L_s equation proposed by Hung et al. (2008) is applicable to cases involving a Cassie-like nanoscale roughened surface. In contrast, in cases with a Wenzel-like nanostructure, the no-slip boundary conditions are independent of variations in the contact angle. The adoption of a Wenzel–Cassie hybrid model helped to verify that the fluid density inside the cavity is a critical indicator of wettability of the wall–fluid interface. Our results also demonstrate that ρ_{f, cav} is a critical property in the measurement of hydrodynamic effects and thus its importance as an indicator of the validity of the equation θ ? L_s. The average time that water molecules are trapped and the number of averaged hydrogen bonds within cavities in a dynamic fluid field were also investigated.

     

Hydrodynamic boundary conditions and dynamic forces between bubbles and surfaces

Dynamic forces between a 50 microm radius bubble driven towards and from a mica plate using an atomic force microscope in electrolyte and in surfactant exhibit different hydrodynamic boundary conditions at the bubble surface. In added surfactant, the forces are consistent with the no-slip boundary condition at the mica and bubble surfaces. With no surfactant, a new boundary condition that accounts for the transport of trace surface impurities explains variations of dynamic forces at different speeds and provides a direct connection between dynamic forces and surface transport effects at the air-water interface.     

Boundary slip on smooth hydrophobic surfaces: intrinsic effects and possible artifacts

We report an accurate determination of the hydrodynamic boundary condition of simple liquids flowing on smooth hydrophobic surfaces using a dynamic surface force apparatus equipped with two independent subnanometer resolution sensors. The boundary slip observed is well defined and does not depend on the scale of investigation from one to several hundreds of nanometers, nor on shear rate up to 5 x 10(3)s(-1). The slip length of 20 nm is in good agreement with theory and numerical simulations concerning smooth nonwetting surfaces. These results disagree with previous data in the literature reporting very high boundary slip on similar systems. We discuss possible origins of large slip length on smooth hydrophobic surfaces due to their contamination by hydrophobic particles.     

Relaxation time and viscosity of water near hydrophilic surfaces

Recently results of microwave spectroscopy have shown that the water relaxation time is lengthened near the surface of aggregates of hydrogen bond-forming molecules, an effect corresponding to an enhanced viscosity [1–3]. In this communication properties of the so-called bound water are treated by a percolation calculation on the water cluster sizes. The values combined with the statistical model of the dielectric response of bulk water by Haggis et al. [4] yield the bound water relaxation time and in this way its viscosity. Fair agreement with the experimental data is obtained. The new analysis represents a keystep for a better understanding of bound water in physico-chemical and biological systems. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

Molecular kinetic theory of boundary slip on textured surfaces by molecular dynamics simulations

A theoretical model extended from the Frenkel-Eyring molecular kinetic theory (MKT) was applied to describe the boundary slip on textured surfaces. The concept of the equivalent depth of potential well was adopted to characterize the solid-liquid interactions on the textured surfaces. The slip behaviors on both chemically and topographically textured surfaces were investigated using molecular dynamics (MD) simulations. The extended MKT slip model is validated by our MD simulations under various situations, by constructing different complex surfaces and varying the surface wettability as well as the shear stress exerted on the liquid. This slip model can provide more comprehensive understanding of the liquid flow on atomic scale by considering the influence of the solid-liquid interactions and the applied shear stress on the nano-flow. Moreover, the slip velocity shear-rate dependence can be predicted using this slip model, since the nonlinear increase of the slip velocity under high shear stress can be approximated by a hyperbolic sine function.     

Percolation transition of hydration water: from planar hydrophilic surfaces to proteins

The formation of a spanning hydrogen-bonded network of hydration water is found to occur via a 2D percolation transition in various systems: smooth hydrophilic surfaces, the surface of a single protein molecule, protein powder, and diluted peptide solution. The average number of water-water hydrogen bonds at the percolation threshold varies from 2.0 to 2.3, depending on temperature, system size, and surface properties. Calculation of nH allows an easy estimation of the percolation threshold of hydration water in various systems, including biomolecules.     

Structural and dynamical properties of water confined between two hydrophilic surfaces

The properties of water in the vicinity of surfaces and under confinement have been extensively studied because of the relevance of a quantitative understanding of many processes that not only take place in biological systems, like cells, membranes and microemulsions, but also in many others such as confined water in rocks, ionic channels and interestellar matter. In this work we perform molecular dynamic calculations of the nanoscopic structure of TIP5P model water confined between two hydrophilic surfaces. We calculate the diffusion coefficients and the atomic density profile of water molecules and polar ions in the system as a function of the number of water molecules per amphiphilic (n_W). We also study the dependence of the water layer thickness and the profiles of water dipole orientation with this parameter.     

Nanodroplets on rough hydrophilic and hydrophobic surfaces

We present results of Molecular Dynamics (MD) calculations on the behavior of liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On hydrophobic surfaces, the contact angle for nanodroplets depends strongly on the root-mean-square roughness amplitude, but it is nearly independent of the fractal dimension of the surface. Since increasing the fractal dimension increases the short-wavelength roughness, while the long-wavelength roughness is almost unchanged, we conclude that for hydrophobic interactions the short-wavelength (atomistic) roughness is not very important. We show that the nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is no contact angle hysteresis due to strong thermal fluctuations, which occur at the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however, there is strong contact angle hysteresis due to higher energy barrier. These findings may be very important for the development of artificially biomimetic superhydrophobic surfaces.     

Atomic force microscopy analysis of wool fibre surfaces in air and under water

Wool fibre surfaces have been treated by solvent cleaning which leaves the native covalently bound surface lipid layer intact, and by alcoholic alkali which removes the lipid layer. The resultant surfaces have been analysed by atomic force microscopy (AFM), with particular emphasis on force–distance (F–d) methods. Methodologies were developed for investigation in situ in water of both the surface topography and the characteristics of the lipid layer. Longitudinal surface texturing was resolved in images of wool fibre surfaces in air; the texturing remained prominent after exposure to water. High resolution F–d curves revealed features associated with the lipid layer. A simple formalism was used to show that the layer had a thickness of a few nm, and an effective stiffness of some 0.12±0.01 N/m. Strong adhesive interactions, equivalent to a pressure of 0.1 MPa, acted on the tip at the tip-to-substrate interface. The methodology and formalism are likely to be relevant in the broad field of thin-film analysis and for fibre technology.     

Co-adsorption of surfactants and propyl gallate on the hydrophilic oxide surfaces

Propyl gallate (PG) adsolubilisation in the cationic, anionic and nonionic surfactant micelles formed in the bulk solution and at the silica/solution interface has been investigated. It was found that in the absence of surfactant, propyl gallate does not adsorb on the silica surface from aqueous solution. However, in the presence of hexyltrimethylammonium bromide (CTAB), its uptake by silica significantly increases. Alumina is quite an effective adsorbent for SDS and propyl gallate and does not adsorb nonionic TX-100. The addition of PG promotes adsorption of SDS and TX-100.     

Rolling resistance moment of microspheres on surfaces: contact measurements

Using contact measurements, experimental evidence was obtained for the existence of the rolling resistance moment. The critical rolling distance prior to detachment is reported. Previously it has been argued that the critical rolling distance should be related to the lattice size and/or the molecular length of the particle and surface materials. However, there has been no theoretical prediction for the critical value and, currently, the reasons for its existence are not fully understood. For polystyrene latex (PSL) particles, measurements presented in the current study on silicon suggest much higher values for the critical rolling distance than previous anticipated levels. The current approach can also be employed to measure the work of adhesion between a spherical particle and a flat surface without the prior knowledge of the particle diameter since the rolling moment stiffness is directly proportional to the work of adhesion with no dependence on the diameter of the particle. Experimental results are compared with the available data and good agreement between the theoretical predictions and the experimental values is found.     

Nanorheology: An investigation of the boundary condition at hydrophobic and hydrophilic interfaces

It has been shown that the flow of a simple liquid over a solid surface can violate the so-called no-slip boundary condition. We investigate the flow of polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a hydrophobic surface made of a Self-Assembled Monolayer of OTS (octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus (DSFA) which allows one to study the flow of a liquid film confined between two surfaces with a nanometer resolution. No-slip boundary conditions are found for both fluids on hydrophilic surfaces only. Significant slip is found on the hydrophobic surfaces, with a typical length of one hundred nanometers. Received 21 December 2001 and Received in final form 3 August 2002 RID="a" ID="a"e-mail: ccottin@dpm.univ-lyon1.fr RID="b" ID="b"Present address.     

Molecular hydrodynamics of inhomogeneous systems: The origin of slip boundary conditions

Molecular hydrodynamic equations derived by linear response theory for inhomogeneous systems are investigated in two extreme cases: a simple fluid in a gravitational field (an example of a long-range inhomogeneity), and a simple fluid in the presence of a rigid flat wall described by an infinite potential step (an example of a short-range inhomogeneity). In both cases the phenomenological equations result when the molecular equations are smoothed over a length which is large compared to the correlation lenght. For these systems the local equilibrium assumption is shown to be valid. In addition it is shown how the usual slip boundary conditions arise as a consequence of the interaction which causes the short-range inhomogeneity.     

Potentials of mean force for adatoms on surfaces

Differences between the adatom-adatom potential of mean force and the pair potential in the limit of low adatom density are illustrated through the Kirkwood equation for a two-component solid. The major difference which remains at all adatom densities arises from the adatom-substrate contribution to the potential of mean force. A method designed to extract an estimate of the adatom-adatom pair potential from field ion microscopy data is suggested.     

Wetting of phospholipid membranes on hydrophilic surfaces - Concepts towards self-healing membranes

We report on the wetting behavior of phospholipid membranes on solid surfaces immersed in aqueous solution. Using fluorescence microscopy, the spreading velocity of fluid bilayers advancing from a lipid source is investigated. The kinetic spreading coefficient was measured as a function of temperature for pure DMPC membranes and as a function of charge density and cholesterol content for binary membranes. A theoretical model for the membrane flow is presented, which takes into account the liquid crystalline bilayer architecture of the lipid membrane. The spreading power results from the membrane-solid VdW interaction and is dissipated in hydrodynamic shear flow as well as by inter-monolayer friction within the bilayer. The frictional drag causes a dynamic tension gradient in the spreading membrane, which is manifested by a single exponential decay of the fluorescence intensity profile along the spreading direction. Obstacles are shown to act as pinning centers deforming the advancing line interface. However, no depinning was observed, since the centers are circumflown without abrupt relaxation. Received 6 November 1998     

Chemisorption,photodesorption and conductivity measurements on ZnO surfaces

Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO₂ is the only photodesorbed species from both the single crystal and powder samples studied. The CO₂ photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO₂ photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO₂^? “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO₂ and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO₂^? “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence.     

Electrostatic force microscopy on oriented graphite surfaces: coexistence of insulating and conducting behaviors

We present measurements of the electric potential fluctuations on the surface of highly oriented pyrolytic graphite using electrostatic force and atomic force microscopy. Micrometric domainlike potential distributions are observed even when the sample is grounded. Such potential distributions are unexpected given the good metallic conductivity of graphite because the surface should be an equipotential. Our results indicate the coexistence of regions with "metalliclike" and "insulatinglike" behaviors showing large potential fluctuations of the order of 0.25 V. In lower quality graphite, this effect is not observed. Experiments are performed in Ar and air atmospheres.     

Chemisorption of water on nickel surfaces

Magnetic and volumetric measurements associated with neutron inelastic spectroscopy demonstrate that water is dissociatively adsorbed on nickel. In the case of Raney nickel, for low coverages, the oxygen is fixed on surface aluminium atoms and hydrogen occupies two nickel sites. At high coverage, the water molecule is fixed via oxygen bonding to the hydrogen covered nickel. On nickel prepared from its hydroxide, adsorption is reversible, the water molecule occupying six metal sites.