AuthorsThoroddsen, Sigurdur T
KAUST DepartmentHigh-Speed Fluids Imaging Laboratory
Mechanical Engineering Program
Physical Science and Engineering (PSE) Division
Online Publication Date2020-01-28
Print Publication Date2020-01-01
Embargo End Date2021-01-28
Permanent link to this recordhttp://hdl.handle.net/10754/661331
MetadataShow full item record
AbstractWe investigate the dynamics of a drop containing a single solid particle impacting on a solid surface. The particle rebounds through the drop during impact and can separate from the deposited liquid above an impact velocity threshold. We show that this threshold can be predicted by a simple energy balance. Moreover, we discover a new type of liquid jetting ejected above the particle faster than the impact velocity. We demonstrate that this jetting is due to the focusing effect of the liquid on the solid substrate below the rebounding particle. Although the wetting properties of the particle have a minor effect on the separation threshold, they play a key role in the liquid jetting by affecting the immersion depth of the particle at the time of impact.
CitationThoroddsen, S. T., & Thoraval, M.-J. (2020). Jetting from an impacting drop containing a particle. Physics of Fluids, 32(1), 011704. doi:10.1063/1.5139534
JournalPhysics of Fluids
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Drop impact splashing and air entrapmentThoraval, Marie-Jean (2013-03) [Dissertation]
Advisor: Thoroddsen, Sigurdur T
Committee members: Chung, Suk Ho; Kasimov, Aslan R.; Quere, David; Samtaney, RaviDrop impact is a canonical problem in fluid mechanics, with numerous applications in industrial as well as natural phenomena. The extremely simple initial configuration of the experiment can produce a very large variety of fast and complex dynamics. Scientific progress was made in parallel with major improvements in imaging and computational technologies. Most recently, high-speed imaging video cameras have opened the exploration of new phenomena occurring at the micro-second scale, and parallel computing allowed realistic direct numerical simulations of drop impacts. We combine these tools to bring a new understanding of two fundamental aspects of drop impacts: splashing and air entrapment. The early dynamics of a drop impacting on a liquid pool at high velocity produces an ejecta sheet, emerging horizontally in the neck between the drop and the pool. We show how the interaction of this thin liquid sheet with the air, the drop or the pool, can produce micro-droplets and bubble rings. Then we detail how the breakup of the air film stretched between the drop and the pool for lower impact velocities can produce a myriad of micro-bubbles.
Partial coalescence from bubbles to dropsZhang, F. H.; Thoraval, Marie-Jean; Thoroddsen, Sigurdur T; Taborek, P. (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2015-10-07) [Article]The coalescence of drops is a fundamental process in the coarsening of emulsions. However, counter-intuitively, this coalescence process can produce a satellite, approximately half the size of the original drop, which is detrimental to the overall coarsening. This also occurs during the coalescence of bubbles, while the resulting satellite is much smaller, approximately 10 %. To understand this difference, we have conducted a set of coalescence experiments using xenon bubbles inside a pressure chamber, where we can continuously raise the pressure from 1 up to 85 atm and thereby vary the density ratio between the inner and outer fluid, from 0.005 up to unity. Using high-speed video imaging, we observe a continuous increase in satellite size as the inner density is varied from the bubble to emulsion-droplet conditions, with the most rapid changes occurring as the bubble density grows up to 15 % of that of the surrounding liquid. We propose a model that successfully relates the satellite size to the capillary wave mode responsible for its pinch-off and the overall deformations from the drainage. The wavelength of the primary wave changes during its travel to the apex, with the instantaneous speed adjusting to the local wavelength. By estimating the travel time of this wave mode on the bubble surface, we also show that the model is consistent with the experiments. This wavenumber is determined by both the global drainage as well as the interface shapes during the rapid coalescence in the neck connecting the two drops or bubbles. The rate of drainage is shown to scale with the density of the inner fluid. Empirically, we find that the pinch-off occurs when 60 % of the bubble fluid has drained from it. Numerical simulations using the volume-of-fluid method with dynamic adaptive grid refinement can reproduce these dynamics, as well as show the associated vortical structure and stirring of the coalescing fluid masses. Enhanced stirring is observed for cases with second-stage pinch-offs. Numerous sub-satellites are observed when the length of the top protrusion of the drop exceeds the Rayleigh instability wavelength. We also find a parameter regime where the focusing of more than one capillary wave can pinch-off satellites. One realization shows a sequence of three pinch-offs, where the middle one pinches off a toroidal bubble.
Antibubbles and fine cylindrical sheets of airBeilharz, D.; Guyon, A.; Li, Erqiang; Thoraval, M.-J.; Thoroddsen, Sigurdur T (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2015-08-14) [Article]Drops impacting at low velocities onto a pool surface can stretch out thin hemispherical sheets of air between the drop and the pool. These air sheets can remain intact until they reach submicron thicknesses, at which point they rupture to form a myriad of microbubbles. By impacting a higher-viscosity drop onto a lower-viscosity pool, we have explored new geometries of such air films. In this way we are able to maintain stable air layers which can wrap around the entire drop to form repeatable antibubbles, i.e. spherical air layers bounded by inner and outer liquid masses. Furthermore, for the most viscous drops they enter the pool trailing a viscous thread reaching all the way to the pinch-off nozzle. The air sheet can also wrap around this thread and remain stable over an extended period of time to form a cylindrical air sheet. We study the parameter regime where these structures appear and their subsequent breakup. The stability of these thin cylindrical air sheets is inconsistent with inviscid stability theory, suggesting stabilization by lubrication forces within the submicron air layer. We use interferometry to measure the air-layer thickness versus depth along the cylindrical air sheet and around the drop. The air film is thickest above the equator of the drop, but thinner below the drop and up along the air cylinder. Based on microbubble volumes, the thickness of the cylindrical air layer becomes less than 100 nm before it ruptures.