Droplet Movements and Continuous Flows in Rough and Hydrophobic Microchannels

Dynamic contact angles on PTFE surface by aqueous surfactant solution in the absence and presence of electrolytes

This work presents the comparison between the change of a dynamic contact angle during drop spreading over copper surfaces obtained in the experiment and calculated by using empirical correlations (Bracke et al., Jiang et al., Seebergh et al.). It is found that these correlations are applicable for the case of drop spreading over a smooth surface or over a rough surface into the low capillary number region (2.5·10 −7 ). Dynamic contact angles obtained experimentally increase with increasing capillary number, besides it increases significantly on more rough surfaces. However the calculated values of angles do not depend on Ca .

The engineering applications of spreading and adhesion phenomena involving fluids on solids are numerous. The adhesive and spreading interactions at the solid-fluid interfaces are well characterized by dynamic contact angles. This study reports on the results of an experimental investigation into the effect of solid surface roughness on dynamic contact angles in solid-liquid-liquid (S-L-L) systems. The experiment involved the use of Wilhelmy Plate apparatus to measure adhesion tension (which is the product of interfacial tension and cosine of the contact angle between the liquid-liquid interface and the solid surface), the DuNuoy tensiometer to measure the liquid-liquid interfacial tension, and a profilometer to characterize the roughness of the solid surfaces used. The components of the solid-liquid-liquid systems studied consisted of: (i) smooth glass, roughened quartz and an actual rock surface for the solid phase, (ii) normal-hexane and deionized water as the two immiscible liquid phases. The dynamic contact angles (advancing and receding angles) of the three-phase (rock-oil-water) system provide essential information about the wettability of petroleum resrvoirs. The wettability of a reservoir is an important parameter that affects oil recovery in primary, secondary, and enhanced recovery operations [1]. Contact angle measurements on smooth surfaces are generally used to characterize reservoir wettability. However pore surfaces within reservoir rocks are essentially rough and hence it is important to determine the effect of such roughness on measured contact angles. There is very little information in the open literature on the effect of surface roughness on dynamic contact angles in S-L-L systems. In the present work, four levels of roughness of solid surfaces of similar mineralogy (quartz and glass) were tested in hexane-deionized water fluid pair. The advancing and receding contact angles measured at ambient conditions were analyzed for wettability effects. It was found that as surface roughness increased, the dynamic contact angles also increased. The wettability of the rock-oil-water system shifted from weakly water-wet for the smooth glass to intermediate-wet for the roughened surface. The general trends observed in our study were found to be in good agreement with other published results. However, the generally held notion of increasing contact angle hysteresis with increasing roughness appears to be incorrect in solid-liquid-liquid systems.

Additive manufacturing (AM) allows for the fabrication of complex metallic specimens which are otherwise very difficult or impossible to create using traditional manufacturing methods. For multi-phase heat spreaders such as vapor chambers, complex internal geometry lends itself well to AM as new design features can be realized. The internal geometries of vapor chambers frequently involve several thin pillars, which act as wicking structures and support a small internal cavity in which the phase change of a liquid, usually water, provides a means to effectively ‘spread’ a high heat flux throughout a larger area. In this case study, Laser Powder Bed Fusion (L-PBF) AM was utilized to create a series of aluminum alloy (AlSi10Mg) support pillars of fixed height with varying filet angles and build orientations (0°, 10°, 20°, 30°, 40°, 50° and 60° from the normal surface) to determine effects on surface roughness and resolution. Surface-to-water contact angles were measured on rinsed, un-polished surfaces, and microscopy was employed to obtain images demonstrating surface roughness profiles. This was done on both heated and unheated surfaces. Results clearly demonstrate that the surface roughness of upward and downward-facing surfaces during L-PBF depends on build orientation. The observational findings of this experiment showed that the surface roughness on surfaces representing horizontally oriented faces decreased as the printing orientation increased. This indicates that one must be cognizant of printing orientation for the L-PBF of capillary structures. A change in surface roughness as a result of printing orientation effects the heat transfer within an enclosed space due to a change in contact angles between the liquid and the heated surface. The results of this study will aid in future design considerations of heat pipes for their additive manufacture via current L-PBF technology.

Evaluation of unusual change in contact angle between MgO and molten magnesium

A study of the structure and surface properties of nanostructured biocompatible coatings on Ti alloys

Rarefied gas flows in rough microchannels are investigated by non-equilibrium molecular dynamics simulations. The surface roughness is modelled by an array of triangular modules. The Maxwell slip model is found to break down due to the surface roughness for gas flows in microchannels with large surface roughness. Non-Maxwell slippage shows that the slip length is smaller than that predicted by the Maxwell model and is nonlinearly related to the mean free path. For larger surface roughness and smaller Knudsen number, the non-Maxwell effect becomes more pronounced. The boundary conditions, generally including velocity slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface roughness. Simulation results show that A/λ ≈ 1 is a good criterion to validate the no-slip boundary condition and A/λ > 0.3 can be a criterion to judge the occurrence of non-Maxwell slippage, where A is the surface roughness size and λ is the mean free path of gas molecules. The permeability enhanced by the surface roughness may be responsible for the roughness-induced non-Maxwell slippage.

Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly developed viscous force formulation at the liquid–solid interface to reproduce the rate-dependent behaviour of the moving contact line. Dynamic contact angle simulations with the interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous forces with different magnitude on the contact angle dynamics is examined by empirical power-law correlations; the derived constants suggest that the dynamic contact angle changes monotonically with the interfacial viscous force. The simulation results are consistent with experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with the slip length of flow and the microscopic surface roughness. This work demonstrates that the modified SPH model can successfully account for the rate-dependent effects of a moving contact line, and can be used for realistic multiphase flow simulations under dynamic conditions.

Using a continuum model capable of describing the one-component liquid-gas hydrodynamics down to the contact line scale, we carry out numerical simulation and physical analysis for the droplet motion driven by thermal singularity. For liquid droplets in one-component fluids on heated or cooled substrates, the liquid-gas interface is nearly isothermal. Consequently, a thermal singularity occurs at the contact line and the Marangoni effect due to temperature gradient is suppressed. Through evaporation or condensation in the vicinity of the contact line, the thermal singularity makes the contact angle increase with the increasing substrate temperature. This effect on the contact angle can be used to move the droplets on substrates with thermal gradients. Our numerical results for this kind of droplet motion are explained by a simple fluid dynamical model at the droplet length scale. Since the mechanism for droplet motion is based on the change of contact angle, a separation of length scales is exhibited through a comparison between the droplet motion induced by a wettability gradient and that by a thermal gradient. It is shown that the flow field at the droplet length scale is independent of the statics or dynamics at the contact line scale.

Spontaneous imbibition in microchannels is a critical phenomenon in various industrial applications, such as enhanced oil recovery and microfluidic systems. One of the key factors influencing the imbibition process is the dynamic wetting effect, which governs the interaction between the liquid and solid surfaces. This paper improves the original pseudopotential model for interfluid forces by coupling it with the Peng-Robinson equation of state. The model's accuracy is verified through thermodynamic consistency checks, simulations of gas-liquid interfacial tensions, and testing of static equilibrium contact angles. Following model validation, we use it to simulate spontaneous gas-liquid imbibition in microchannels and investigate dynamic contact angle evolution during the process. The results demonstrate that (1) as the microchannel width increases, inertia forces become more significant during the initial imbibition stages, leading to a greater difference between the dynamic and static contact angles. (2) A decrease in fluid-solid interaction strength results in a larger gap between dynamic and static contact angles. (3) Higher interfacial tension strengthens the capillary forces, accelerating the imbibition rate and enlarging the difference between the dynamic and static contact angles. Furthermore, the dynamic contact angle data obtained from our simulations can be used to correct the traditional Lucas-Washburn equation. The corrected equation predicts imbibition distances that closely match the simulation results.

Surface control of hydrophilicity and degradability with block copolymers composed of lactide and cyclic carbonate bearing methoxyethoxyl groups

Changes in dynamic contact angle for the wetting of heat-treated paraffin wax surfaces were reported. It was observed that the advancing angle remains relatively stable while the receding angle decreases with greater treatment. These changes appear to result primarily from a reduction in crystallinity, which was gauged via differential scanning calorimetry and X-ray diffraction. Surface roughness is also found to decrease with heat treatment, but its role in the reported phenomenon appears to be minimal. This is demonstrated by normalizing the roughness of coated plates with cold pressing. Along with global changes in crystallinity, modifications to the local crystallinity were observed using Raman confocal microscopy. Specifically, it is shown that the heat treatment increased the heterogeneities of the wax surfaces by producing micrometer-scale amorphous regions. These regions are believed to act as hydrophilic defects on the wetted surface, impacting almost exclusively receding angle measurements. The findings raise concerns over the use of semicrystalline oligomeric and polymeric materials as model substrates in wetting experiments. Results also identify a possible method for estimating the order of packing of straight-chain hydrocarbon derivatives at surfaces using dynamic contact angle measurements.

The static shape of droplets under electrowetting actuation is well understood. The steady-state shape of the droplet is obtained on the basis of the balance of surface tension and electrowetting forces, and the change in the apparent contact angle is well characterized by the Young-Lippmann equation. However, the transient droplet shape behavior when a voltage is suddenly applied across a droplet has received less attention. Additional dynamic frictional forces are at play during this transient process. We present a model to predict this transient behavior of the droplet shape under electrowetting actuation. The droplet shape is modeled using the volume of fluid method. The electrowetting and dynamic frictional forces are included as an effective dynamic contact angle through a force balance at the contact line. The model is used to predict the transient behavior of water droplets on smooth hydrophobic surfaces under electrowetting actuation. The predictions of the transient behavior of droplet shape and contact radius are in excellent agreement with our experimental measurements. The internal fluid motion is explained, and the droplet motion is shown to initiate from the contact line. An approximate mathematical model is also developed to understand the physics of the droplet motion and to describe the overall droplet motion and the contact line velocities.

The effect of lead-manganese naphthenate on the change in the dynamic contact angle during spreading of a styrene-acrylic dispersion is studied. The dependences of the change in the dynamic contact angle in time are obtained for various concentrations of lead-manganese naphthenate in solution. As a result of the analysis of the experimental data, a concentration region was revealed in which the dynamic wetting angle varies in the widest range with the lowest speed compared to solutions of other concentrations. The results of the research will be used to optimize the composition of the polymer-mineral composite material and will make it possible to obtain a polymer matrix that most wetts the mineral fillers, and the resulting composite will have the greatest mobility and vitality.

The Effect of Drop (Bubble) Size on Advancing and Receding Contact Angles for Heterogeneous and Rough Solid Surfaces as Observed with Sessile-Drop and Captive-Bubble Techniques