Weng Kong Chan

Eccentricity effect of micropatterned surface on contact angle

This article experimentally shows that the wetting property of a micropatterned surface is a function of the center-to-center offset distance between successive pillars in a column, referred to here as eccentricity. Studies were conducted on square micropatterns which were fabricated on a silicon wafer with pillar eccentricity ranging from 0 to 6 μm for two different pillar diameters and spacing. Measurement results of the static as well as the dynamic contact angles on these surfaces revealed that the contact angle decreases with increasing eccentricity and increasing relative spacing between the pillars. Furthermore, quantification of the contact angle hysteresis (CAH) shows that, for the case of lower pillar spacing, CAH could increase up to 41%, whereas for the case of higher pillar spacing, this increment was up to 35%, both corresponding to the maximum eccentricity of 6 μm. In general, the maximum obtainable hydrophobicity corresponds to micropillars with zero eccentricity. As the pillar relative spacing decreases, the effect of eccentricity on hydrophobicity becomes more pronounced. The dependence of the wettability conditions of the micropatterned surface on the pillar eccentricity is attributed to the contact line deformation resulting from the changed orientation of the pillars. This finding provides additional insights in design and fabrication of efficient micropatterned surfaces with controlled wetting properties.

Surface-tension-driven liquid–liquid displacement in a capillary

This paper presents theoretical and experimental studies of the surface-tension-driven, two-immiscible liquid?liquid displacement in a horizontal capillary. Using the dynamic contact angle approach, a one-dimensional mathematical model is developed to describe the capillary displacement of a fixed liquid column by another liquid that continuously flows into the capillary by surface tension. Experiments for a water column displaced by silicon oil were carried out to examine the effect of the water column length and capillary diameter on the displacement dynamics. It was found that a faster and longer displacement is achieved for a shorter water column in a larger capillary. The theoretical predictions agree reasonably well with the experimental studies conducted for the capillary displacement of the silicon oil?water interface. In addition, for a single liquid capillary flow, dimensionless Pi groups are derived using the similarity analysis. It is demonstrated that using the derived Pi groups, the experimental data for the displacement of silicon oil of various viscosities in capillaries of various diameters can collapse into one curve that is consistent with the Washburn equation.

An Improved Scoring Matrix for Multiple Sequence Alignment

The way for performing multiple sequence alignment is based on the criterion of the maximum-scored information content computed from a weight matrix, but it is possible to have two or more alignments to have the same highest score leading to ambiguities in selecting the best alignment. This paper addresses this issue by introducing the concept of joint weight matrix to eliminate the randomness in selecting the best multiple sequence alignment. Alignments with equal scores are iteratively rescored with the joint weight matrix of increasing level (nucleotide pairs, triplets, and so on) until one single best alignment is eventually found. This method for resolving ambiguity in multiple sequence alignment can be easily implemented by use of the improved scoring matrix.

Eccentricity effects of microhole arrays on drag reduction efficiency of microchannels with a hydrophobic wall

This paper experimentally investigates the effects of microhole eccentricity on the slip lengths of Stokes flow in microchannels with the bottom wall made of microhole arrays. The wettability of such microhole structures fabricated by the replica molding of polydimethylsiloxane is first analyzed measuring both static and dynamic contact angles. Subsequently, the drag reduction performance of the microchannels with such hydrophobic microhole surfaces is evaluated. The results indicate that the impact of microhole eccentricity on drag reduction performance correlates well with the contact angle hysteresis rather than with the static contact angle. Furthermore, microhole arrays with large normalized width and zero eccentricity show the minimum contact angle hysteresis of 18.7°. In these microchannels, the maximum percentage increase in the relative velocity is 39% corresponding to a slip length of 2.49 μm. For the same normalized width, increasing the normalized eccentricity to 2.6 increases the contact angl...

Characterization of surface tension and contact angle of nanofluids

This paper investigates the effects of nanoparticles on surface tension and equilibrium contact angle of TiO2 - DI water nanofluids. Experimental measurements of surface tension by using the pendant droplet method show that the surface tension of the TiO2 - DI water nanofluids depends weakly on nanoparticle concentration; however, at higher nanoparticle concentrations the surface tension is lower. Various mechanisms are reported to explain this behavior. Experimental measurements of contact angles of the TiO2 - DI water nanofluids droplets on borosilicate glass slides exhibit strong nanoparticle dependence, and the general trend is increment of the contact angles with nanoparticle concentration. The effect from the so-called disjoining pressure due to the presence of nanoparticles within the thin nanofluid film wedge at the vicinity of the three-phase contact line is examined. However, the phenomenon is attributed to the pinning of contact line and local changes in solid-liquid interfacial tension due to the depositing of nanoparticles on adsorption sites on solid surface.

The three-phase contact line shape and eccentricity effect of anisotropic wetting on hydrophobic surfaces

This paper experimentally evaluates the combined effects of eccentricity, relative spacing, and viewing directions on the wetting conditions and the three-phase contact line shapes of hydrophobic surfaces patterned with discrete micropillars. Different techniques to depict the tortuosity of the contact line between the water droplet and microstructured surfaces are presented. First, square micropillars with different values of normalized eccentricity, e*, and relative spacing, D*, were fabricated using a double casting replication technique. Subsequently, the contact angles were measured along different viewing angles by gradually rotating the sample from 0° to 180°. The contact angle distribution was found as a periodic function of the viewing angle whose period depends on the micropillar eccentricity. The results showed that anisotropy increases by increasing the micropillar eccentricity or decreasing the pillar relative spacing. However, the effect of changing the micropillar eccentricity was much more pronounced. Micropillars with e* = 0.75 and smaller D* showed maximum degrees of anisotropic wetting and droplet distortion corresponding to 7% and 15%, respectively. Using the measured droplet aspect ratio, corrugated shapes of the three-phase contact line of the micropillars were also reconstructed. Finally, a simple yet effective semi-analytical model, based on Fourier series curve-fitting of the experimental data, was developed to describe the equilibrium 3D shape of the droplet on anisotropic surfaces. Experimental and simulation results reveal that the degrees of anisotropic wetting and droplet distortion were directly proportional to the energy barriers of the system, resulting from the noncircular corrugated shape of the three-phase contact line. The obtained results may further shed light on the underlying mechanism influencing anisotropic wetting on micropatterned surfaces.

Dynamic contact angle of water-based titanium oxide nanofluid

This paper presents an investigation into spreading dynamics and dynamic contact angle of TiO2-deionized water nanofluids. Two mechanisms of energy dissipation, (1) contact line friction and (2) wedge film viscosity, govern the dynamics of contact line motion. The primary stage of spreading has the contact line friction as the dominant dissipative mechanism. At the secondary stage of spreading, the wedge film viscosity is the dominant dissipative mechanism. A theoretical model based on combination of molecular kinetic theory and hydrodynamic theory which incorporates non-Newtonian viscosity of solutions is used. The model agreement with experimental data is reasonable. Complex interparticle interactions, local pinning of the contact line, and variations in solid–liquid interfacial tension are attributed to errors.

Analytical modeling of ultra-thin-film bearings

We develop an analytical model based on kinetic theory of gases and we apply this to thin-film bearings that are frequently encountered in data storage systems. This model incorporates the effects of the higher-order stress tensor and temperature gradient in the governing equations and it can be used for a wide range of Knudsen numbers covering the complete slip regimes. The corresponding modified Reynolds equation is obtained and the flow rate is plotted versus the inverse Knudsen number. Pressure distributions along the air bearing at various Knudsen numbers, bearing numbers, Mach numbers and pitching angles are presented and the load carrying capacities are also obtained. The results are compared with the first-order and second-order slip models. It is observed that the current work is in very good agreement with solutions obtained from the linearized Boltzmann equation and direct simulation Monte Carlo results.

Fluid Mechanics of Flow Through Rectangular Hydrophobic Microchannels

In this study, the effect of two important parameters have been evaluated for pressure driven liquid flows in microchannel in laminar regime by analytical modeling, followed by experimental measurement. These parameters are wettability conditions of microchannel surfaces and aspect ratio of rectangular microchannels. For small values of aspect ratio, the channel was considered to have a rectangular cross-section, instead of being two parallel plates. Novel expressions for these kinds of channels were derived using eigenfunction expansion method. The obtained two-dimensional solutions based on dual finite series were then extended to the case of a constant slip velocity at the bottom wall. In addition, for large values of aspect ratio, a general equation was obtained which is capable of accounting for different values of slip lengths for both upper and lower channel walls. Firstly, it was found out that for low aspect ratio microchannels, the results obtained by analytical rectangular 2-D model agree well with the experimental measurements as compared to one dimensional solution. For high aspect ratio microchannels, both models predict the same trend. This finding indicates that using the conventional 1-D solution may not be accurate for the channels where the width is of the same order as the height. Secondly, experimental results showed that up to 2.5% and 16% drag reduction can be achieved for 1000 and 250 micron channel height, respectively. It can be concluded that increasing the surface wettability can reduce the pressure drop in laminar regime and the effect is more pronounced by decreasing the channel height.Copyright

Fluid Velocity Slip and Temperature Jump at a Solid Surface

A comprehensive review of current analytical models, experimental techniques, and influencing factors is carried out to highlight the current challenges in this area. The study of fluid-solid boundary conditions has been ongoing for more than a century, starting from gas-solid interfaces and progressing to that of the more complex liquid-solid case. Breakthroughs have been made on the theoretical and experimental fronts but the mechanism behind the phenomena remains a puzzle. This paper provides a review of the theoretical models, and numerical and experimental investigations that have been carried out till date. Probable mechanisms and factors that affect the interfacial discontinuity are also documented.