Ri Li

Dynamics of ice nucleation on water repellent surfaces.

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.

Characteristics of liquid sheets formed by two impinging jets

An experimental investigation of liquid sheets formed by the impingement of two capillary liquid jets is conducted. The breakup mechanism of the sheet is categorized into two main regimes and five subregimes based on the experimental observations. Two types of Reynolds numbers (jet Reynolds number and sheet Reynolds number) are introduced to correlate with and map the reported breakup regimes. Breakup length and width are introduced, and their analytical models are derived. Both the analytical and experimental results show that the breakup length and width are linearly proportional to the Weber number of individual jets in the case of closed-rim sheets. The slopes of the two linear relations are dependent on the impinging angle. The distribution of fluid velocity in the sheet is examined and found to be in disagreement with the assumption of early models, which claims uniform velocity across the sheet. The nonuniform distribution of fluid velocity in the sheet causes a discrepancy between analytically pre...

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.

Some observations for effects of copper on zinc phosphate conversion coatings on aluminum surfaces

Abstract X-Ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and electrochemical measurements have been used to characterize zinc phosphate (ZPO) conversion coatings, formed on high-purity aluminum surfaces, after dipping in coating baths containing different concentrations of Cu2+ up to 50 ppm. Significant variations in the morphology, adhesion and corrosion protection afforded by the ZPO coatings are especially observed as the Cu2+ in the bath varies from 0 to 10 ppm. Comparisons are made with the effects of surface Cu segregation on ZPO coatings formed in a previous study made on Al-2024-T3 alloy.

Surface effects in chromate conversion coatings on 2024-T3 aluminum alloy

Chromate conversion coatings formed on samples of 2024-T3 aluminum alloy, which had been given different pre-treatments, were examined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and corrosion tests. Two pre-treatments were considered, namely a simple mechanical polish, and polishing followed by an etch in a HF-H2SO4 solution. The latter treatment leads to significant Cu enrichment at the oxide-alloy interface, and this in turn can lead to a deleterious effect on the corrosion protection afforded by a subsequently applied chromate coating. Discussions are given of the behaviour of Cu in the coating formed on the sample that received an acid etch in the pre-treatment. This involves both migration through the coating and a non-uniform redeposition of Cu on to the coating surface. By contrast, the sample that initially was given just the mechanical polish in the pre-treatment does not show a Cu enrichment in the surface region, and the subsequently applied coating appeared stable after a 24 h immersion in a NaCl test solution.

Drawback During Deposition of Overlapping Molten Wax Droplets

Two overlapping droplets impacting on a solid surface coalesce and recoil so that the edges of the droplets are drawn back, a phenomenon called drawback. A series of experiments were conducted on the merging of two overlapping wax droplets deposited on an aluminum drum to characterize the drawback process between the two droplets. Drum temperature, droplet overlap ratio, and the time interval between impacts of droplets were varied. Wax bumps, formed by coalescence of two droplets on the drum surface, were photographed and their length and width measured. An aspect ratio and dimensionless drawback index, quantifying the extent of drawback, were calculated from these measurements. When drum temperature is increased, or the time interval between impacts of the two droplets is reduced, there is more drawback and the ink bumps become round, since the cooling rate of droplets is slower and droplets have a longer time to change shape due to surface tension. A simple heat transfer model was developed to predict changes in droplet-cooling rate with changes in droplet overlap, substrate temperature, or time interval (deposition frequency). Experiments were also conducted on the formation of lines by depositing 20 droplets. Measurements on the drawback of two droplets were used to predict conditions under which broken lines are obtained.

Impact of a single drop on a flowing liquid film.

The impact of a single liquid drop on a flowing liquid film is experimentally and theoretically studied. The drop impact produces a crownlike rising liquid sheet, which radially expands. Small droplets can be formed from the crown sheet, resulting in splash. The present study results in three major contributions. (1) A theoretical model is developed to predict the expansion of the crown base. The model with an introduced energy loss factor is shown to be in satisfactory agreement with our experimental observations of drop impact on both stationary and flowing films. The energy loss factor is correlated to the properties of the film and drop. (2) Analysis is conducted to derive an equation for evaluating the stretching rate of the rising crown sheet, which is the local gradient of the rising velocity at the top edge of the crown sheet. It shows that the highest stretching rate appears where the drop spreading flow is right opposite to the film flow, which helps explain why the same location is most probable for splash to take place. (3) A parameter as a function of modified Weber and Reynolds numbers is defined to predict splash and nonsplash of drop impact on flowing films. The two nondimensional numbers evaluate the competition of the two flows (drop and film) against viscosity and surface tension effects. A threshold value of the parameter is found for the occurrence of splash impact on flowing films.

Contact Angle and Local Wetting at Contact Line

This theoretical study was motivated by recent experiments and theoretical work that had suggested the dependence of the static contact angle on the local wetting at the triple-phase contact line. We revisit this topic because the static contact angle as a local wetting parameter is still not widely understood and clearly known. To further clarify the relationship of the static contact angle with wetting, two approaches are applied to derive a general equation for the static contact angle of a droplet on a composite surface composed of heterogeneous components. A global approach based on the free surface energy of a thermodynamic system containing the droplet and solid surface shows the static contact angle as a function of local surface chemistry and local wetting state at the contact line. A local approach, in which only local forces acting on the contact line are considered, results in the same equation. The fact that the local approach agrees with the global approach further demonstrates the static contact angle as a local wetting parameter. Additionally, the study also suggests that the wetting described by the Wenzel and Cassie equations is also the local wetting of the contact line rather than the global wetting of the droplet.

CHARACTERIZATION OF ALUMINUM SURFACES AFTER DIFFERENT PRETREATMENTS AND EXPOSURE TO SILANE COUPLING AGENTS

X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to characterize surfaces of aluminum which have been pretreated by mechanical polishing, acid etching and alkaline etching, as well as given subsequent exposures to air and water. These surfaces can differ markedly with regard to their chemical compositions and topographical structures. Characterizations of these surfaces after exposures to three organosilanes, γ-GPS, BTSE and γ-APS, indicate that the amount of silane adsorbed in each case shows a tendency to increase both with the number of OH groups detected at the oxidized aluminum and with the surface roughness. The XPS data are consistent with the adhesion of γ-APS occurring through H bonding, especially via NH3+ groups.

Maximum Spread of Droplet on Solid Surface: Low Reynolds and Weber Numbers

This theoretical study proposes an analytical model to predict the maximum spread of single droplets on solid surfaces with zero or low Weber and Reynolds numbers. The spreading droplet is assumed as a spherical cap considering low impact velocities. Three spreading states are considered, which include equilibrium spread, maximum spontaneous spread, and maximum spread. Energy conservation is applied to the droplet as a control volume. The model equation contains two viscous dissipation terms, each of which has a defined coefficient. One term is for viscous dissipation in spontaneous spreading and the other one is for viscous dissipation of the initial kinetic energy of the droplet. The new model satisfies the fundamental physics of drop-surface interaction and can be used for droplets impacting on solid surfaces with or without initial kinetic energy.