An evaporation model for oscillating spheroidal drops

The evaporation process of a liquid spheroidal drop floating in a gaseous atmosphere has been modelled, accounting for the oscillation between oblate and prolate states. A previously developed exact solution for the heat and mass transfer equations has been extended to investigate the effect of oscillation on drop evaporation under the assumption of quasi steady-state conditions and the results are compared with approximate models from the open literature. The validity of the quasi steady-state assumption is discussed, deriving, for different fluids, the range of drop temperature and size and gas temperature where it is reasonably acceptable

Drop Impact on Liquid Surfaces: Formation of Lens and Spherical Drops at the Air-Liquid Interface

Droplets at the air-liquid interface of immiscible liquids usually form partially-submerged lens shapes (e.g. water on oil). In addition to this structure, we showed that droplets released from critical heights above the target liquid can sustain the impact and at the end maintain a spherical ball-shape configuration above the surface, despite undergoing large deformation. Spherical drops are unstable and will transform into the lens mode due to slight disturbances. Precision dispensing needles with various tip diameter sizes were used to release pendant drops of deionized water onto the surface of fluorocarbon liquid (FC-43, 3M). A cubic relationship was found between the nozzle tip diameter and the released droplet diameter. Drop impact was recorded by a high speed camera at a rate of 2000 frames per second. In order for the water drops to sustain the impact and retain a spherical configuration at the surface of the target liquid pool, it is required that they be of a critical size and be released from a certain height; otherwise the commonly observed lens shape droplets will form at the surface

Single droplet impacts onto deposited drops. Numerical analysis and comparison

The impact of a spherical water droplet onto a stationary sessile droplet lying on a solid wall is studied numerically using the volume-of-fluid methodology. The governing Navier-Stokes equations are solved both for the gas and liquid phase coupled with an additional equation for the transport of the liquid interface. An unstructured numerical grid is used along with an adaptive local grid refinement technique, which enhances the accuracy of the numerical results along the liquid-gas interface and decreases the computational cost. The stationary sessile droplet has been created from the prior impact of one or two water droplets falling onto the solid wall, while two solid walls have been studied−an aluminum substrate and a glass substrate. The material of the wall plays an important role because it has an impact on the droplet's wetting behavior. The numerical model is validated against corresponding experimental data presented in the first part of the present work (Nikolopoulos et al., 2010), showing good agreement. Furthermore, the numerical investigation sheds light on the governing physics of the phenomenon

Wetting behavior in the inertial phase of droplet impacts onto sub-millimeter microstructured surfaces

Hypothesis: The shape of the wetted region after a droplet impact can vary significantly even in the early phase of the process. How much of the early spreading process occurs within the structures versus above the structures, flow regimes and local wetting at groove intersections can have effects on the sizes and shapes of the final wetted regions. Experiments and simulations: We experimentally study droplet impacts onto cubic pillars with [Formula presented], [Formula presented] and [Formula presented] side length, height and separation. Weber numbers ranged between 80 and 1 100, while Reynolds numbers varied between 1 150 and 10 600 using water and isopropanol droplets. The contact angle on a flat segment of the samples was modified between θFS<5∘ and θFS=120∘±5∘. Several experiments are reproduced using our in-house code FS3D to show the internal flow fields. Findings: Diamond-shaped spreading patterns with edges aligned at 45∘ to the structure pattern are observed. A transition between top-dominated (circular) spreading and diamond spreading occurs depending on the structure size and impact velocity. Groove intersections can act as flow dividers, causing spreading along a path with 90∘ bends. For large structure sizes and impact velocities fluid jets can pass through the structures uninhibited

Proceedings of the DIPSI Workshop 2022: Droplet Impact Phenomena & Spray Investigations, Bergamo, Italy, 1st July 2022

This Book of Proceedings contains the extended abstracts of the contributions presented at the DIPSI Workshop 2022 on Droplet Impact Phenomena and Spray Investigation, organised by the University of Bergamo on Friday 1st July 2022 in Bergamo, Italy. This workshop, which is now at its fourteenth edition, represents an important opportunity to share the recent knowledge on droplets and sprays in a variety of research fields and industrial applications. The event is supported by the Department of Engineering and Applied Sciences of the University of Bergamo and the Research Training Group 2160/2 DROPIT in collaboration with the University of Stuttgart

Gravity induced shape effects on the time-dependent evaporation of pendant drops

The paper presents a method to model the time-dependent evaporation of pendant drops taking into account the effect of drop deformation induced by gravity. The model is based on the solution to the time-dependent drop mass and energy conservation equations, where the mass and energy fluxes through the gas mixture are numerically evaluated for a range of Bond numbers and contact angles. The evaporation characteristics of pendant and sessile drops on hydrophobic and hydrophilic substrates are compared in terms of evaporation times and evaporative cooling, for both constant contact angle and constant contact radius modes