Drop-on-Drop Impact Dynamics on a Superhydrophobic Surface

Abstract

Investigation of multiple droplet interactions is vital due to its importance in various applications, including sprays. In this context, the interaction of a sessile droplet (droplet #1) interaction with an oncoming droplet (droplet #2), in a vertically aligned, dropon-drop configuration, on a superhydrophobic surface is investigated, both experimentally and computationally. Three droplet impact regimes, with low We number of the impacting droplet (i.e., We droplet #2 ∼ 1.0) is observed, viz., gentle merging, late merging, and droplet bounce-off without merging. Complementary simulation of the gentle merging regime shows that a very high-pressure gradient is generated between the neck and the bulk region immediately after the contact, which acts as a driving force for the subsequent rapid evolution of the droplet shape. With a larger We number (We droplet #2 ∼ 8.2 and 14.8), there is always a unique outcome of the impact experiments. The two droplets merge, spread, recede, and bounce-off; there are multiple instances of rebound of the combined droplet mass from the surface. The estimated shear stress emanating during the impact and subsequent dynamics is large enough to cause deterioration of chemically coated superhydrophobic surfaces. The difference in the shape of moving droplet #2 at the instance of impact for We droplet #2 ∼ 1, as compared to We droplet #2 ∼ 8.2 and 14.8 is also highlighted. Important time scales during coalescence and beyond are the inertial-capillary time scale (∼16 ms) and viscous-capillary time scale (∼8 s). We also highlight the role of the droplet #1 deposition boundary condition, influence of the We number of droplet #2, and substrate wettability on drop-on-drop interactions, which has direct relevance to the transport mechanisms happening in various engineering applications like spraying pesticides, spray cooling, and rain interactions, etc.

Successive Rebounds of Impinging Water Droplets on Superhydrophobic Surfaces

“…As a consequence, the droplet would bead up as a pearl with an equilibrium contact angle ( θ eq ) of >150° and a contact angle hysteresis (i.e., the difference between advancing contact angle θ a and receding contact angle θ r ) of <10° . This so-called Cassie–Baxter state can be maintained even under dynamic loading, where water droplets impinging on superhydrophobic surfaces with certain velocities would eventually rebound off. − The bouncing behavior of impinging water droplets has been considered as a defining characteristic of superhydrophobic surfaces , and is also the key feature in robust applications such as waterproofing and self-cleaning textiles, anti-icing materials, and surface charge printing for droplet transport and electricity generation via raindrops. , …”

Effect of Sodium Carboxymethyl Cellulose on the Dynamic Wetting Characteristics of the Dust Suppression Droplet Impacting the Coal Surface

“…The existing research shows that the main factors affecting the droplet dynamic wetting behavior include impact velocity, impact angle, droplet size, physical and chemical properties of the droplet (viscosity, surface tension, etc. ), surface wettability and solid surface roughness, and so forth. − For example, Han et al studied the dynamic wetting behavior of underwater oil droplets impacting the smooth brass substrate and 316L stainless steel using a high-speed camera technology. It was found that with the increase of dimensionless time, the dimensionless spread length of the droplet had a negative correlation with the contact angle .…”

Effect of Preparation Processes on Structural Thermal Stability and Collision Energy Dissipation of Common Antirelaxation Coatings on Quartz Substrates

“…As a result, the relaxation rate starts increasing instead. In the same way, the dense and staggered quartz surface coated with such a coating will be prone to induce a cascade effect of elastic collision between the AFM probe tip and the atoms on coating surface; 46 that is, a single cycle collision does not signify a single collision only. Additionally, the uncertainty around the spatial distribution of its Young's modulus and molecular orientation makes the rebound angle of the AFM probe tip random after each collision, increasing the compensation voltage for restoring the AFM system to resonance.…”