Fluid-surface Interaction Research Articles

Evaluation of low frequency polarization models using well characterized sintered porous glass samples

Recently, significant progress has been made in understanding low frequency complex conductivity measurements of rocks. The relevant publications study these methods in two different ways: On the one hand, petrophysical interpretation has been improved in terms of theoretical approaches. These approaches are either modifications of pore size related membrane polarization concepts or rely on grain size related polarization models in combination with semi-empirical model function parameterizations. On the other hand, extensive petrophysical studies provide insight in dependencies of IS parameters on structural and electrochemical rock properties. We aim at assessing the results of published theoretical and experimental findings for a reference system, consisting of sintered porous glass samples. Thereby, we benefit from well characterized samples, which allow for direct tests of theoretical predictions and empirical relations. We find that: (1) The correlation σ″~Sm is stronger than σ″~Spor for a wide range of fluid conductivities and frequencies above 1Hz. (2) Correlation coefficients for the imaginary conductivity to inner surface area relations are strongly frequency dependent. (3) Normalized chargeability, obtained by fitting a Cole–Cole model to the spectral data, provides a fair alternative to single frequency information. (4) Correlation breaks down for disturbed fluid–surface interaction by wettability manipulation. (5) Salinity dependence of proportionality factors a1=Sm/σ″ and a2=Spor/σ″ due to a salinity dependent partition coefficient is confirmed qualitatively. Quantitative theoretic predictions of a1 or a2 fail due to the assumption of non-reduced Stern layer mobility for clay free silica. (6) Earliest grain size related models provide the best quantitative estimate of relaxation time. (7) Results agree well with published data for sands and sandstones with respect to (i) quantitative estimates of a1 or a2 and (ii) influences of rock structural parameters on relaxation time.

Numerical simulation of condensation on structured surfaces.

Condensation of liquid droplets on solid surfaces happens widely in nature and industrial processes. This phase-change phenomenon has great effect on the performance of some microfluidic devices. On the basis of micro- and nanotechnology, superhydrophobic structured surfaces can be well-fabricated. In this work, the nucleating and growth of droplets on different structured surfaces are investigated numerically. The dynamic behavior of droplets during the condensation is simulated by the multiphase lattice Boltzmann method (LBM), which has the ability to incorporate the microscopic interactions, including fluid-fluid interaction and fluid-surface interaction. The results by the LBM show that, besides the chemical properties of surfaces, the topography of structures on solid surfaces influences the condensation process. For superhydrophobic surfaces, the spacing and height of microridges have significant influence on the nucleation sites. This mechanism provides an effective way for prevention of wetting on surfaces in engineering applications. Moreover, it suggests a way to prevent ice formation on surfaces caused by the condensation of subcooled water. For hydrophilic surfaces, however, microstructures may be submerged by the liquid films adhering to the surfaces. In this case, microstructures will fail to control the condensation process. Our research provides an optimized way for designing surfaces for condensation in engineering systems.

Influence of Substrate Elasticity on Droplet Impact Dynamics

Droplet impact dynamics is vital to the understanding of several phase-change and heat-transfer phenomena. This work examines the role of substrate elasticity on the spreading and retraction behavior of water droplets impacting flat and textured superhydrophobic substrates. Experiments reveal that droplet retraction on flat surfaces decreases with decreasing substrate elasticity. This trend is confirmed through a careful measurement of droplet impact dynamics on multiple PDMS surfaces with varying elastic moduli and comparison with impact dynamics on hard silicon surfaces. These findings reveal that surfaces tend to become more wettable upon droplet impact as the elastic modulus is decreased. First-order analyses are developed to explain this reduced retraction in terms of increased viscoelastic dissipation on soft substrates. Interestingly, superhydrophobic surfaces display substrate-elasticity-invariant impact dynamics. These findings are critical when designing polymeric surfaces for fluid-surface interaction applications.

Nanoscale elastoplastic adhesion of wet asperities

Accurate prediction of friction is crucial for design and efficient operation of many devices, comprising various contacts. In practice, contacting surfaces are rough and often wet. There are several mechanisms, which contribute to friction, including viscous shear of a coherent fluid film, as well as that of a thin adsorbed layer of boundary active molecular species. Additionally, adhesion and elastoplastic deformation of asperities on counterface surfaces may occur. Traditional friction models are based on statistical representation of surface topography as well as description of boundary shear films based on the theoretical lubricant film Eyring shear stress. The study reports a more realistic friction model than the traditional ones, which do not take into account the wet nature of the asperities. The fluid–surface interaction is a main contribution of the article, not hitherto reported in literature. It is shown that ignoring the effect of surface wetness can lead to the over-estimation of boundary friction and under-estimation of contact load-carrying capacity.

Temperature dependent droplet impact dynamics on flat and textured surfaces

Droplet impact dynamics determines the performance of surfaces used in many applications such as anti-icing, condensation, boiling, and heat transfer. We study impact dynamics of water droplets on surfaces with chemistry/texture ranging from hydrophilic to superhydrophobic and across a temperature range spanning below freezing to near boiling conditions. Droplet retraction shows very strong temperature dependence especially on hydrophilic surfaces; it is seen that lower substrate temperatures lead to lesser retraction. Physics-based analyses show that the increased viscosity associated with lower temperatures combined with an increased work of adhesion can explain the decreased retraction. The present findings serve as a starting point to guide further studies of dynamic fluid-surface interaction at various temperatures.

Extension of the Steele 10-4-3 potential for adsorption calculations in cylindrical, spherical, and other pore geometries

Simplified fluid-substrate interaction models derived from the Lennard-Jones potential are widely used in the simulation of gas physisorption phenomena. In this paper, we reinterpret the well known Steele 10-4-3 potential for a gas molecule interacting with a planar surface, and use the resultant scheme to derive new potentials for cylindrical and spherical pore geometries. These new potentials correctly recover the Steele result in the limit of infinite pore radius, a useful improvement over existing models. We demonstrate the new cylindrical Steele 10-4-3 potential in calculations of argon adsorption via fluid density functional theory. This potential yields markedly different adsorption behavior than existing cylindrical potentials, which follow from small but significant differences in both the strength and the shape of the fluid-surface interaction. These differences cannot be fully reconciled simply by reparameterizing (scaling) the existing models; the new potential is more realistic in design, and is especially to be preferred in studies where comparison with planar substrates is made. Finally, we discuss extensions of this approach to more complicated pore geometries, yielding a family of Steele-like potentials that all satisfy the correct planar limit.

The initial development of a jet caused by fluid, body and free surface interaction. Part 4. The large-time structure

The free surface and flow field structure generated by the uniform acceleration of a rigid plate, inclined at an angle α ∈ (0, π) to the exterior horizontal, as it advances into an initially stationary and horizontal strip of inviscid incompressible fluid, is studied in the large-time limit via the method of matched asymptotic expansions. The small-time limit of the solution to this problem has been studied in detail in Needham et al. (2008, The initial development of a jet caused by fluid, body and free-surface interaction. Part 3. An inclined accelerating plate. Q. Jl. Mech. Appl. Math., 64, 581–615). We assume that after an initial constant acceleration, the velocity of the plate reaches a terminal value in finite time and remains at that velocity thereafter. The structure of the large-time solution includes a steady bore propagating away from the plate with constant speed, together with an algebraically decaying wave field behind the bore, up to the plate, which demonstrates that there are no localized trapped waves excited near to the plate, with all energy radiating away from the plate as t → ∞.

The use of analytic continuation to increase the accuracy in modelling fluid–surface interactions in cylindrical nanopores

In this paper, the mathematical formulation of a rigid/smooth-walled semi-empirical fluid–surface interaction potential is discussed. Potentials of this type may be used to model the adsorption of simple fluids in cylindrical nanopores, for example, cavities in a solid material or free standing single- or multi-wall nanotubes. Analysis of the properties of the hypergeometric series functions used to evaluate these potentials indicates that modelling of the fluid–surface interactions for particles very near to the surface is improved with analytically continuous series. The derivation and implementation of a potential representing a cylindrical cavity within a continuous solid material are also presented.

Surface aided replica exchange algorithm applied to the prewetting transition

A novel algorithm, termed surface aided replica exchange (SARE), was introduced in which the fluid–surface interaction was varied in order to generate different replicas in a particular ensemble. Exchange between replicas was allowed with a probability of acceptance obtained by imposing detailed balance. The method was implemented in a modified isothermal-isobaric ensemble that permitted precise characterization of the prewetting line of a simple adsorbed fluid. The prewetting line for each surface was characterized by computing the adsorption of the fluid as a function of pressure, and the wetting temperature estimated for each surface.

To Wet or Not to Wet: That Is the Question

Wetting transitions have been predicted and observed to occur for various combinations of fluids and surfaces. This paper describes the origin of such transitions, for liquid films on solid surfaces, in terms of the gas-surface interaction potentials V(r), which depend on the specific adsorption system. The transitions of light inert gases and H2 molecules on alkali metal surfaces have been explored extensively and are relatively well understood in terms of the least attractive adsorption interactions in nature. Much less thoroughly investigated are wetting transitions of Hg, H2O, heavy inert gases and other molecular films. The basic idea is that nonwetting occurs, for energetic reasons, if the adsorption potential’s well-depth D is smaller than, or comparable to, the well-depth ε of the adsorbate-adsorbate mutual interaction. At the wetting temperature, T w, the transition to wetting occurs, for entropic reasons, when the liquid’s surface tension is sufficiently small that the free energy cost in forming a thick film is sufficiently compensated by the fluid-surface interaction energy. Guidelines useful for exploring wetting transitions of other systems are analyzed, in terms of generic criteria involving the “simple model”, which yields results in terms of gas-surface interaction parameters and thermodynamic properties of the bulk adsorbate.

Simulation of fluid flow in hydrophobic rough microchannels

Surface effects become important in microfluidic setups because the surface to volume ratio becomes large. In such setups the surface roughness is not any longer small compared to the length scale of the system and the wetting properties of the wall have an important influence on the flow. However, the knowledge about the interplay of surface roughness and hydrophobic fluid–surface interaction is still very limited because these properties cannot be decoupled easily in experiments.We investigate the problem by means of lattice Boltzmann (LB) simulations of rough microchannels with a tunable fluid–wall interaction. We introduce an ‘effective no-slip plane’ at an intermediate position between peaks and valleys of the surface and observe how the position of the wall may change due to surface roughness and hydrophobic interactions.We find that the position of the effective wall, in the case of a Gaussian distributed roughness, depends linearly on the width of the distribution. Further, we are able to show that roughness creates a nonlinear effect on the slip length for hydrophobic boundaries.

The initial development of a jet caused by fluid, body and free surface interaction. part 3. an inclined accelerating plate

The free surface and flow field structure generated by the uniform acceleration of a rigid plate, inclined at an angle a ∈(0, π/2) U (π/2, π) to the exterior horizontal, as it advances into an initially stationary and horizontal strip of inviscid, incompressible fluid, are studied in the small-time limit via the method of matched asymptotic expansions. This work generalises the case of a uniformly accelerating vertical plate, when a = π/2, as studied in King and Needham (J. Fluid. Mech. 268 (1994)). Particular attention is devoted to the inner region in the vicinity of the intersection point between the plate and the free surface. It emerges that the angle a = π/2 is a bifurcation point in this local structure. For a ∈ (0, π/2), a weak jet rises up the plate when t = 0 + , with thickness 0(t 2 ) as t → 0 + , independent of a, with the free surface slope at the plate being O (t π α-2 ) as t → 0 + ; this slope is O(1/log(t)) as t → 0 + when a = π/2. However, when a ∈ (π/2, π), the jet becomes significantly stronger, with a highly nonlinear structure, and the thickness now depending on and increasing with a, being O (t γ ), where γ = (1-π/4α) -1 . In this case, moreover, a classical solution to the evolution problem is possible only when a ∈ (π/2, α c ], where α c ≈ 1.791 ≈ 102.6°. When a = α c , a 120° comer forms on the free surface when t = 0 + at the initial intersection point of the plate and free surface, and convects self-similarly into the inner region for 0 < t « 1. We conjecture that no classical solution exists when t = 0 + for plate angles a ∈(α c , π). In practice, surface tension allows a solution to exist in some finite neighbourhood of t = 0, a result that will be presented in a later paper.

Micro-scale liquid–liquid separation in a plate-type coalescer

The plate-type coalescer was constructed and tested for the continuous separation of immiscible liquids in micro-scale. Two plates formed the main structure of the coalescer. A flat and rectangular channel was situated between the plates. The material options for the plates were PTFE, stainless steel and glass which formed the contact surfaces for the fluids in the device. The liquid–liquid dispersion generated by the standard slit interdigital micromixer was fed in the coalescer where the coalescing of the droplets occurred. The coalescer had two outlets, one for the aqueous phase and the other for the organic phase enabling the continuous separation and withdrawal of separated phases. The performance of the coalescer was evaluated with respect to the channel height, flow rate, residence time and plate configuration. It was observed that in suitable conditions, the plate-type coalescer offered the efficient phase separation continuously under stable operation. The separation method utilizes the interaction between the fluids and channel surface. The flat channel, 100 or 200 μm, enabled the fluid–surface interaction. By using PTFE and stainless steel materials as channel surfaces and the total flow rate less than 180 ml/h, the stable operation and complete separation of aqueous and organic phases were possible. One hundred and eighty millilitres per hour corresponds to superficial velocity of 3.3 cm/s in 100 μm height channel and 1.7 cm/s in 200 μm height channel. The phase separation in the coalescer took place considerably faster than conventionally in a decantation container.

Computer simulations of wetting of solid surfaces by liquid crystals

Atomistic computer simulations are presented, consisting of droplets of model liquid crystal molecules wetting a generic crystalline surface. It is shown that the type of wetting that occurs is highly dependent on the value of the surface interaction parameter epsilon{fs} , switching between partial wetting (no spreading) for epsilon{fs}=2.0 to complete wetting for epsilon{fs}=2.5 . epsilon{fs} is the multiplicative factor which scales the well depth of the fluid-surface interaction energy. During complete wetting, the spreading occurs through the growth of a precursor layer, and a set of secondary terraces. The temperature affects the structure of the droplet, as might be expected when different phases are sampled, and also the shape of the spreading droplet, and the rate of spreading. In particular, in the smectic-A phase at 350K, for values of epsilon{fs} just large enough to induce complete wetting, the precursor film assumes a diamond shape, with edges normal to the [110] directions in the crystal surface. It is shown that spreading occurs through diffusion across the surface, with the radius of the precursor layer increasing with the square root of time. Mass flow studies indicate that the spreading occurs by molecules cascading over the top of the droplet to feed the growing precursor layer. Calculations of contact angle relaxation are in qualitative agreement with experimental findings.

Drying layer near a weakly attractive surface

Depletion of the liquid density near a solid surface with a weak long-range fluid–surfaceinteraction was studied by computer simulations of the liquid–vapour coexistence of aLennard-Jones (LJ) fluid confined in slitlike pores. In a wide temperature range the liquiddensity decreases towards the surface without the formation of a vapour layer between theliquid and the solid surface. This evidences the absence of a drying transition upto the liquid–vapour critical point. Two contributions to the excess desorptionΓtot were found.The first one, Γξ∼ρbulkξ, exists at any temperature and diverges as the bulk correlation lengthξ when approaching the liquid–vapour critical temperatureTc. The secondcontribution, ΓL∼ρbulkL0, originates from a microscopic drying layer near the solid boundary. At high temperatures the thicknessL0 of the drying layer increases in accordance with the power lawL0∼−ln(1−T/Tc), indicating adrying transition at Tc. The drying layer can be suppressed by strengthening the fluid–surface interaction,by increasing the fluid–surface interaction range or by decreasing the pore size.

Boundary slip as a result of a prewetting transition

Some fluids exhibit anomalously low friction when flowing against a certain solid wall. To recover the viscosity of a bulk fluid, slip at the wall is usually postulated. On a macroscopic level, a large slip length can be explained as a formation of a film of gas or phase-separated “lubricant” with lower viscosity between the fluid and the solid wall. Here we justify such an assumption in terms of a prewetting transition. In our model the thin-thick film transition together with the viscosity contrast gives rise to a large boundary slip. The calculated value of the slip length has a jump at the prewetting transition temperature which depends on the strength of the fluid-surface interaction (contact angle). Furthermore, the temperature dependence of the slip length is nonmonotonous.

Flow regulation in microchannels via electrical alteration of surface properties

The development of microfluidic (lab-on-a-chip) technology requires local control of fluid flow in the microchannels. Conventional microvalve approaches involve moving parts and/or complicated fabrication techniques, which makes them unreliable and prevents inexpensive integration in microanalytical systems. We have developed a simple low cost method for regulating fluid flow in microchannels that is compatible with existing microfabrication techniques and eliminates the need for moving parts. We use an electrical signal to stimulate silver deposition on a thin solid electrolyte layer in a small region of a microchannel. Since fluid flow is dominated by the nature of the channel surface, the electrodeposited silver changes the fluid–surface interaction and the effect can be used to control the movement of the fluid. Increases in the contact angles of both water and methanol, by 20 ∘ and 27 ∘ respectively, have been demonstrated. Such changes in hydrophobicity are sufficient to retard or stop capillary or external pressure-driven fluid flow in typical microchannels.

Operational limits for reusable space transportation systems due to physical boundaries of C/SiC materials

The paper concentrates on the surface interaction phenomena of both SiC-coated and uncoated C/C–SiC material of DLR's well known liquid siliconizing process (LSI) while exposing samples and structures to simulated and real space vehicle reentry conditions. During reentry, physical limits for these types of CMC materials are defined by oxidation resistance, passive–active transition conditions and erosion effects by various reasons like ‘mechanical’ fluid–surface interaction. Special emphasis is given to the passive–active transition behaviour of SiC and the related ‘temperature jump effect’ which will be explained by new theoretical research results. In addition a quantitative evaluation of the effect is given based on experimental test sample data. The considerations given in the paper are checked against literature data, observations and experience which have been achieved so far by manifold tests with samples in various facilities, experimental space reentry tests with samples on Russian FOTON capsules and a test with a hot structure during the EXPRESS mission (experiment CETEX). Finally, the understanding of the mechanisms may lead to a revised strategy for optimizing criterions for reentry trajectories of reusable space transportation systems.

Spectral and instantaneous flow field characteristics of the laminar to turbulent transition in a cone and plate apparatus

Fluid-surface interaction, is very much influenced by the flow distribution and the flow spectra. For biological surfaces, cell functions such as mytosis and cell turnover, can be triggered by the instantaneous flow fluctuations which induce augmented shear stress levels inside the wall surface boundary layer. The objective of this work is to study the flow field along a cellular surface and to understand the interaction process. For that purpose, a cone and plate apparatus was built in which the transitional and turbulent instantaneous flow field characteristics, especially near the plate surface, were investigated using spatial hot wire anemometry, concentrating on time domain and spectral analysis. The frequency spectrum of velocity fluctuations near the plate is influenced by the plate roughness. We found that there is a linear relation between wall roughness and the preferred frequencies of the spectra. In addition a universal law exists for mean velocities, similar to the logarithmic law of the wall, when normalized by R˜ 1/2, the apparatus Reynolds number.

A molecular theory for surface forces adhesion measurements

Surface forces have been measured by others in undersaturated vapors to determine the adhesive energy of the solid (mica) as well as to probe the limits of the Laplace-Kelvin equation in micropores. The measured pull-off forces are complex requiring an intimate understanding of the underlying oscillatory solvation forces, adsorption, and surface deformation. While the elastic energy of the solid has been taken into account in previous theoretical studies of adhesion, the Laplace-Kelvin assumption of a uniform bulk-like fluid has always been applied. In this paper we present the first application of a modern molecular theory—a nonlocal density functional theory—to the prediction of pull-off forces with the surface forces apparatus. In this theory, the confined fluid is allowed to be nonuniform, and oscillatory solvation forces may be predicted. For rigid surfaces, it is demonstrated that the separation of forces most often used to analyze the surface forces apparatus measurements is highly accurate only when adsorption is properly treated and when the relative pressure is p/po&amp;gt;0.2−0.4. The limiting value of the relative pressure decreases as the strength of the fluid-surface interaction increases. In addition, the range over which the vacuum limit of the solid surface free energy, γs may be measured is strongly dependent on the strength of various molecular interactions. We predict, as observed in experiments, that the saturation limit of the pull-off force is given by the Laplace pressure alone if there are at least two fluid layers between the surfaces. Finally, we show that using pull-off forces to test the limits of the Laplace-Kelvin theory is misleading because the measurements by design minimize solid-liquid contributions to the total force.