<p>Capturing videos using a high-speed camera, droplet impact upon a hydrophilic micro-pillared substrate was analyzed. Viscosities of the fluids were varied using differing concentrations on glycerol in DI water and ranged from Ohnesorge number 0.0023-0.0280. Droplet diameter was kept 2.7mm through all experiments, and the droplet was impacted upon micro-pillars possessing diameter = 52μm, spacing = 21μm, and heights of 65, 100, and 120μm. To study jet evolution, impact velocity was varied in addition to viscosity. While jetting phenomenon occurs in a broad range of scenarios and has been well studied, examination of jet dynamics after droplet impact on a hydrophilic surface has not been as common. Therefore, this paper aims to connect similarities between this form of capillary jetting and another well-known phenomenon of bubble bursting. Bubble bursting stems from the advancement of a capillary wave, while droplet impact causes jetting due to an inertially driven radial flow. Both experience a cavity collapse when opposing fluid fronts collide which causes a singularity on the liquid surface, thus leading to the occurrence of jetting. With the presence of this singularity due to cavity collapse, among other characteristics, the idea holds that jet evolution, and therefore scaling laws, should be quite similar between the two cases. Following processes used to define scaling laws for bubble bursting, a similar approach was taken to derive scaling laws for dimensionless jet height, jet radius, base height, and radius of the jet base for droplet impact on a hydrophilic substrate. The development of a top droplet before breakup from the jet also allows the examination of a scaling law for the necking diameter. In addition to these scaling laws, the relationship between viscosity and spreading diameter and correlations to whether a jet will occur were also explored. Previously determined scaling laws for bubble bursting discovered that the scaling coefficient for most dimensions examined during jet evolution was 2/3. Results from studying droplet impingement so far have shown that the necking diameter fits this scaling coefficient well while other dimensions do not. Jet tip radius was not found to have as dramatic of change and typically had a scaling coefficient of 0.1-0.2. Jet height grew more dramatically, having a coefficient around 1. Nondimensionalized axial velocity and radius of the top droplet had better agreeance; -1/2 and 1 for droplet impact and -3/4 and 5/4 for bubble bursting, respectively.</p>

Jet Dynamics Associated With Drop Impact on Microstructured Hydrophilic Substrates

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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