•Six different surfaces from superhydrophilic to superhydrophobic surfaces are fabricated.•Droplet impact experiments are conducted on such surfaces.•Droplet spreading and height oscillation are ...investigated and analyzed.•The spreading stage is divided into inertial and viscous spreading stages.•A correlation for evolutions of spreading factors on different surfaces is developed.
Six different surfaces ranging from superhydrophilic to superhydrophobic were fabricated. Droplet impact experiments were conducted on these surfaces with water droplets 2.25 mm in diameter at a Weber number close to zero to study the effects of surface wettability on the impact process and post-impact oscillation. Droplet impact on all surfaces, except for superhydrophobic surfaces, is dominated by a spreading stage and no evident retraction is observed. Using the inertia-capillarity time tc = ρR03/σ as characteristic time, the spreading stage on all surfaces at We < 0.5 was calculated to be 2.25 ± 0.11tc; further, it could be divided into inertial and viscous spreading stages at 1 ± 0.11tc based on droplet height evolution. A semi-empirical correlation for calculating the evolution of spreading factors on different surfaces was fitted by a piecewise exponential function using experimental data, and a deviation of ±20% was observed between the fitted and experimental data.
Surface roughness is an important factor that affects dynamic wetting behavior, which can improve the surface hydrophobicity, so it is of great significance to obtain a better understanding of ...roughness effect from both theoretical and practical perspectives. In this paper, we studied the influence of macro-size surface roughness on contact angle hysteresis and spreading work and analyzed the relationship between contact angle hysteresis and spreading work. Results showed that as the surface roughness increased, both the advancing contact angle and the receding contact angle continued to increase until their maximum values were reached, and then started to decrease within the range of surface roughness studied, while the contact angle hysteresis presented the opposite trend. In addition, with the increase of surface roughness the spreading work initially increased to a certain maximum value, then continuously decreased to the minimum value, and then began to increase within the range of the surface roughness studied. These trends could be attributed to the surface wetting state (Wenzel state, Cassie state, and transition state) changing with the change of surface roughness. These findings can provide guidance for the preparation of wetted surfaces with specific functions, especially when it is required to change the wettability without changing the surface chemical properties.
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Hypothesis: It has been verified that a surface of single micro-scale structures with certain roughness could exhibit petal effect. That is, water drops with a contact angle larger ...than 150° would pin on the petal effect surface. It is conjectured that the water drop could pin on the single micro-scale roughness petal effect surface by totally infiltrating into spaces (or grooves) between micro-pillars.
Experiments: An inverted optical microscopy system is synchronically applied in the process of advancing/receding contact angle (ACA/RCA) measurements to directly observe the wetting behavior of water droplets on hydrophobic patterned surfaces with regular arrays of square micro-pillars.
Findings: A sequence of wetting behavior evolution, Wenzel → petal → pseudo-lotus → lotus, is observed on the hydrophobic patterned surfaces along with increasing surface roughness. It is interesting to observe a Cassie-Wenzel transition for water drops on a petal substrate during the ACA measurement (embedded needle method), leading to two ACAs, one before (in Cassie state) and one after the transition (in Wenzel state). Thus, the petal substrates have large contact angle hysteresis (CAH) (with both ACA and RCA in Wenzel state) to pin the water drop in Wenzel state. A Cassie-Wenzel transition is consistently observed during the evaporation process of water drops on pseudo-lotus substrates, leading to two RCAs: one in Cassie state and one in Wenzel state. The pseudo-lotus substrates have CAH (with both ACA and RCA in Cassie state) small enough to make water drops easily slide off.
The impact of surface nanoscale physical heterogeneity on the wettability of polymeric membranes is still elusive. Conventional wettability analysis includes quantifying the membrane surface ...roughness using AFM followed by measuring the apparent equilibrium contact angle of DI water over the membrane surface. Here, we present a novel experimental approach, solely based on contact angle analysis, to elucidate the impact of surface heterogeneity on the wettability of dense polymeric membranes. The proposed approach involves evaluation of equilibrium and advancing contact angles of at least three non-polar liquids over the membrane surface. Using this information, the wettability parameters including the surface roughness-ratio, frictional pinning force, and the dispersive surface tension component of the polymeric membranes were successfully quantified. The comparison with the conventional approach showed that there are 10%–20% discrepancies between the AFM-based and contact angle-based wettability parameters. The results revealed that the AFM measurements strongly depend on the size of the scanned area, particularly for samples with large surface heterogeneity. Furthermore, the water flux decline due to colloidal fouling was found to be in good agreement with the results of our proposed model. This study can provide new insights into developing advanced membrane materials with desired surface wettability and antifouling property.
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•Surface physical heterogeneity of dense polymeric membranes is determined.•The new method uses solely contact angle measurements.•Non-polar probe liquids are employed to minimize the polar interactions.•The new approach provides fast, reliable, and simultaneous predication of surface roughness-ratio and surface tension.
•Change in θa and spreading radius with pressure are presented.•Comparison between θa and θe for four different substrates are discussed.•Initial contact angle and the minimum spreading ratio change ...with pressure.•Non-dimensional numbers (Re, We, and Ca) change with the pressure increase.•Theoretical and experimental results show good validation.
The dynamics of the hydrodynamically driven droplet are studied on four different substrates at surrounding gauge pressures ranging from 0 to 20 MPa. By combining the extended Overall Energy Balance (OEB) approach, the Lucas empirical model for estimating drop viscosity at elevated pressures, the advancement of a droplet on a solid substrate is modeled as it undergoes constant mass flux addition while maintaining a spherical cap. The theoretical governing equation, which incorporates two different modeling methods of viscous dissipation: lubrication approximation and boundary layer approximation, is validated with an experimental investigation involving hydrodynamically driven water droplets. The results show that an increase in surrounding pressure lowers the spreading radius and simultaneously increases the advancing contact angle. In addition, the minimum spreading ratio falls, and the average Reynolds number decreases in a monotone fashion while the Weber and Capillary numbers rapidly increase with increasing pressure.
•Hematite and quartz surfaces with different roughness were conditioned with starch and DDA.•Snap-in force of water droplets on the surfaces of the conditioned minerals was measured.•The largest ...variation in snap-in force was presented on the roughest surface.•Increasing roughness could enlarge the snap-in force difference for the two minerals.•There is a liner correlation between snap-in force and advancing contact angle.
Surface roughness plays a vital role in flotation, because it has a pronounced impact on colloidal interactions, i.e., solid-liquid adhesion. In the previous studies, the natural mineral surfaces with various roughness were mainly taken as research object and the adhesion force of liquid on mineral surfaces with various roughness after conditioning with surfactants remains unknown. To fulfil this gap, in this study, the role of surface roughness in the snap-in force (An attractive force when the water droplet sharply and instantaneously spreads on the mineral surface in less than 0.1 s) of water droplets on hematite and quartz surfaces after conditioning with soluble starch and dodecylamine (DDA) was investigated using a high-sensitivity microelectronic mechanical balance. Results revealed that the effect of surface roughness on the snap-in force of water droplets on hematite surfaces was distinctly affected by the concentrations of both DDA and soluble starch, while the effect of surface roughness on the snap-in force of water droplets on quartz surfaces was only significantly affected by DDA concentration. After conditioning with 2 × 10−4 mol/L soluble starch and 4 × 10−4 mol/L DDA under pH value of 8.75, increasing surface roughness increased the snap-in force of water droplets on hematite surfaces and decreased the snap-in force of water droplets on quartz surfaces, suggesting that increasing surface roughness could enlarge the difference in the snap-in force of water droplets on the surfaces of the two minerals. In addition, the snap-in force was found to be a linear correlation with the advancing contact angle, providing evidence that surface roughness could affect the wettability and thereby the snap-in force.
•A new variable solid-fluid interaction strength scheme is incorporated to a multicomponent/multiphase lattice Boltzmann method to simulate contact angle hysteresis.•Droplet sliding on a inclined ...isothermal non-ideal/ideal flat plate is simulated showing the decrease/increase in receding/advancing contact angle as the droplet slides along the inclined plate.•Droplet evaporation on a horizontal heated non-ideal flat plate is simulated showing constant contact line and constant contact angle periods.•Good agreements with the previous results are shown.
A variable solid-fluid interaction strength scheme compatible with the lattice Boltzmann method is introduced to investigate contact angle hysteresis phenomena numerically. This method is applied to study two problems: (i) droplet sliding on inclined isothermal plates and (ii) droplet evaporating on horizontal heated plates. It is demonstrated that this method is capable of reproducing the decrease in the receding contact angle and the increase in the advancing contact angle during droplet sliding on an inclined isothermal plate, and the constant contact line (CCL) and constant contact angle (CCA) evaporation periods during droplet evaporation on a horizontal heated plate. These simulated results are in agreement with previous experimental investigations.
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The wetting behavior of an electrolyte solution on the separator, determined by contact-angle measurements, has a significant effect on the internal resistance of the battery and on ...its cycle life. The solvent, the lithium-salt type and its concentration may affect the wettability. However, few systematic studies address the effect of salt concentration on surface tension and contact angle.
Surface tensions and advancing contact angles were measured for dimethyl sulfoxide (DMSO), propylene carbonate (PC), dimethyl carbonate (DMC), and a PC/DMC mixture (1:1 mass ratio) with various concentrations of a lithium salt (LiClO4, LiPF6, and LiTFSI) at 23 °C. Measurements were made by a Krüss Drop Shape Analyzer 100, with a video camera mounted on a microscope to record the drop image.
For DMSO, PC and PC/DMC, surface tensions increase by adding LiClO4 or LiPF6 but decrease upon addition of LiTFSI. For DMC, the lithium salts have little impact on the surface tensions. For each solvent, contact angles and adhesion energies follow the same trend as those for surface tensions. The TFSI- anion reduces the surface tension of the solvent, favoring good wettability of the separator. The optimal surface tension for wettability of Celgard 2500 is at or below 26.1 mN/m.
•The additives in oil chemisorb to steel at 100 °C.•Chemisorbed additives significantly reduce the wetting with oil at 100 °C.•Dynamic parameters are more appropriate than static describing wetting ...with oil.•Additive chemical structure significantly effects the wetting behaviour of oil.
In lubrication, additives are added to base oils to form surface layers that crucially improve its performance, particularly at high temperature. However, very little is known about their influence on the wetting behaviour of oil, even at room temperature, while for high-temperature, where the effects of the additives are the most intense, is absent. Accordingly, this work focuses on the additives’ effects on the wetting of oil on steel at 100 °C, which is relevant for the activity of most oil additives. Simple organic friction modifiers with different numbers of polar head groups, non-polar tail chain length, head-group polarity and saturation were investigated. The results show that additive-films change the steel surface, making it more oleophobic. Additives with one polar head group decrease the wettability by 98% more than the additives with two polar groups, the most-polar fatty acid by 75% more than the least-polar amine, the longest chain length of 18C atoms by 55% more than the shortest chain of 11 C atoms, and the unsaturated additive by 12% more than the saturated additive.
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