Session HH: Drops V: Drops and Surfaces

On the collision and mixing of water droplets on superhydrophobic surfaces

The dynamics of water drop collisions on superhydrophobic surfaces is investigated using high-speed photography. Teflon is sanded to create the superhydrophobic surfaces. The results of the surface fabrication technique are presented, showing the effect of grit size on hysteresis. This method of creating superhydrophobic surfaces allows for the specification of varied advancing contact angles with similar hysteresis, or varying hysteresis with near similar advancing contact angles. Deionized water droplets are made to collide on these surfaces by propelling one droplet into another using a burst of pressurized air. The subsequent collision is captured, and several impact characteristics are calculated as a function of contact angle hysteresis. The Weber number and impact number are calculated, as well as the maximum deformation of the combined drop. In some experiments, the drops left the surface after collision even with low hysteresis at the low Weber numbers tested. Characteristic images of different regimes of the collision dynamics will be presented, as will how each of these regimes affect the mixing of the drops.   

Spontaneous Jumping of Coalescing Drops on a Superhydrophobic Surface

When micrometric drops coalesce in-plane on a superhydrophobic surface, a surprising out-of-plane jumping motion was observed. Such jumping motion triggered by drop coalescence was reproduced on a Leidenfrost surface. High-speed imaging revealed that this jumping motion results from the elastic interaction of the bridged drops with the superhydrophobic/Leidenfrost surface. Experiments on both the superhydrophobic and Leidenfrost surfaces compare favorably to a simple scaling model relating the kinetic energy of the merged drop to the surface energy released upon coalescence. The spontaneous jumping motion on water repellent surfaces enables the autonomous removal of water condensate independently of gravity; this process is highly desirable for sustained dropwise condensation.   

Scaling of anisotropic droplet shapes on chemically stripe-patterned surfaces

We present experimental results on the tunable anisotropic wetting behavior on chemically patterned anisotropic surfaces. The equilibrium shape of asymmetric glycerol droplets, arising from patterns of alternating hydrophilic (pristine SiO$_{2})$ and hydrophobic (fluoroalkylsilane self-assembled monolayers) stripes with dimensions in the low-micrometer range, are investigated in relation to the stripe widths. Owing to the well-defined small droplet volume, the equilibrium shape as well as the observed contact angles exhibit unique scaling behavior. Only the relative width of hydrophilic and hydrophobic stripes proves to be a relevant parameter. Our results on morphologically flat, chemically patterned surfaces show similarities with those of experiments on topographically corrugated substrates. They are discussed in terms of the energetics at the liquid-solid interface. \\[4pt] [1] O. Bliznyuk, E. Vereshchagina, E.S. Kooij, B. Poelsema, Phys. Rev. E \textbf{79 }(2009) 041601   

Initial spreading kinetics of high-viscosity droplets on anisotropic surfaces

Liquid droplets on chemically patterned surfaces consisting of alternating hydrophilic and hydrophobic stripes exhibit an elongated shape [1]. To assess the kinetics we present experimental results on the spreading of glycerol droplets on such surfaces using a high-speed camera. Two spreading regimes are observed, expressed in terms of the time-dependent droplet base diameter which can be described by a r(t) $\propto $ t$^{n}$ power law. Initially, in what is referred to as the inertial regime, the kinetics is dominated by the liquid, and spreading is only weakly dependent on the specific surface properties. As such, liquid spreading is isotropic and the contact line maintains a circular shape. Our results reveal a remarkably long inertial regime, as compared to previous results and available models. Subsequently, in the viscous regime, interactions between the liquid and underlying pattern govern the dynamics. The droplet distorts from a spherical cap shape to adopt an elongated morphology that corresponds to the minimum energy configuration on stripe-patterned surfaces. \\[4pt] [1] O. Bliznyuk, E. Vereshchagina, E.S. Kooij, B. Poelsema, Phys. Rev. E 79 (2009) 041601   

Numerical studies of relationship between splash and dynamic contact angle

We numerically studied splashes on dry surfaces in terms of dynamic contact angle. Our simulation model based on CLSVOF (Coupled level set and Volume-of-Fluid) method, CIP (constraint interpolation profile) method and VISAM3 (Volume/Surface Integrated Average based Multi-Moment Method) has capability to simulate splashes in droplet impact onto flat solid surface. In our simulations, we varied only dynamic advancing contact angle as all other parameters are fixed including equilibrium contact angle. The numerical results show that, as the dynamic advancing contact angle increases, the outer rim of the liquid lamella becomes thick and it seems the number of fingers reduces. On the other hand, we only varied equilibrium angle. However, the equilibrium angel hardly affects the behavior of splashes. We believe that dynamic advancing contact angle is one of key parameters of splashing. We also compared the numerical results with several existing experimental data.   

Influence of Liquid Type on Drop Impingement on Rib and Cavity Superhydrophobic Surfaces

We report results of an experimental investigation of liquid drops impinging on superhydrophobic surfaces. The surfaces are fabricated in Silicon wafers with micro-ribs and cavities (grooves) that are coated with a fluoropolymer or teflon hydrophobic coating. Liquid droplets of known size were dropped from heights ranging from 0.5 to 50 cm onto the surfaces and the pre-impingement freefall, surface impact, and droplet deformation were imaged at a rate of 6000 frames/second with a digital camera. The droplets were either water, ethanol, or a glycerine/water mixture. The droplet impact speed, maximum droplet spread, horizontal spread speed, vertical speed of the issuing jet, and the time between impact and formation of the issuing jet were all characterized. The results show that the overall impact dynamics are strongly influenced by the different impinging surface conditions and the fluid type. Results were compared with previously proposed analytical models and suggestions for improving those models are made.   

Insight into drop runback on hydrophilic to superhydrophobic surfaces by shearing airflow

Drop runback has many diverse applications including airfoil icing and fuel cell flooding. In this talk, we use surface science and fluid dynamics principles to explain incipient runback for a drop exposed to shearing airflow. Through experiments with single drops of water and hexadecane (0.5-100 $\mu $l) on PMMA, Teflon, and a superhydrophobic aluminum surface (SHS), wetting parameters such as surface tension, drop shape and contact angle are found to be major controllers of the minimum required air velocity for drop shedding. Exponential functions are proposed that relate air velocity to drop base length and projected area. By normalizing the results, the three water systems can be collapsed to a single curve that also explains results from other researchers, vastly increasing predictive power. SHS are seen to shed drops more easily compared to the other surfaces, with evidence that the drops roll along the surface instead of sliding. Using high speed video, oscillating drop shape and variation of contact angles are also analyzed as they change with air and drop speed.   

The Effect of Hydrogen Passivation Surface on Silicon Nanodroplets Coalescence

Understanding a fundamental formation mechanism of nanoparticles growth and controlling primary particle size and extent of agglomeration when grown from the gas--phase are the significant challenges in the use of nanoparticles. In this talk a possibly mathematical model to describe the droplet coalescence is presented. Here the coalescence of hydrogen terminated silicon surface slowing the process has been studied, and results are compared with molecular dynamics simulations. Nanodroplets of the size between 2 and 6 \textit{nm} at 1500 K were considered. The hydrogen passivation surface completely changes and slows the beginning of the coalescence process. In addition, the presence of hydrogen atoms reduces surface tension of the droplet about 40 to 50{\%}. The model is able to describe both initial induction period and the standard coalescence period. It presents that the effective surface tension decreases with increasing hydrogen coverage, making it harder for droplets to coalesce. http://www.ou.edu/mms/   

Two-dimensional droplet spreading over random topographical substrates

We examine theoretically the effects of a random topographical substrate on the motion of a two-dimensional droplet by developing appropriate statistical approaches. Our theory is based on a set of integro-differential equations for the two droplet fronts, previously obtained for deterministic substrates through a singular perturbation method. We provide a stochastic representation of random substrates as families of certain stationary random functions parametrized by a characteristic amplitude and a characteristic wavenumber. The droplet footprint is found to be a normally distributed random variable as it evolves towards equilibrium. The statistical analysis of the droplet shift along the substrate is highly non-trivial, but its variance can be deduced theoretically at early times and in the long-time limit. It is shown that substrate roughness tends to decrease the wetting properties of the droplet and that its approach to equilibrium is significantly slower for the droplet shift than its footprint, suggesting that the droplet has the tendency to slide without spreading along the substrate features in search for equilibrium. Our theoretical predictions are verified by numerical experiments.   

Effect of contact angle and humidity on evaporation of inkjet-printed colloidal drops

Inkjet printing has attracted much attention in recent years due to its ability to dispense precise amounts of functional materials onto targeted areas. Although evidence exists for a multi-stage evaporation of a sessile drop, the actual evaporation behavior of an inkjetted colloidal drop is not well understood. In this study, a novel visualization technique is developed wherein aqueous suspensions of fluorescent particles are inkjetted onto transparent surfaces and the evaporation dynamics are observed in real-time using a high-power microscope. Two influencing parameters, the ambient humidity and substrate wettability, are systematically varied. It has been confirmed that jetted drops follow a \textit{pinned}, \textit{dewetting}, and \textit{mixed }multi-stage evaporation process. The results also show that the relative humidity acts mainly to accelerate or decelerate the process whereas its relationship to contact angle is not as direct. Contact angle hysteresis plays an important role in controlling the initial pinned mode. For lower contact angle substrates, evaporation drives a flow of particles to deposit near the contact line which set the conditions for the dewetting stage that follows. Finally, a diffusion-controlled evaporation model is used to predict the time internals for each evaporation stage. The model agrees well with the experimental data, especially for the dewetting mode.