Beads-assisted Investigation of Self-Coalescence of Sessile Water Drops (original) (raw)
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ANALYSING DELAYED COALESCENCE OF WATER DROPLETS AND ITS POSSIBLE MECHANISM
Using reliable data from certain referenced papers(see end of paper), we try to model the phenomenon of delayed coalescence and hence explain(mostly qualitatively) the time lag and related effects involved in the process. Drops of water of radii 1mm were dropped from various heights on a water container. For a certain range of heights, the water drop took longer to coalesce and for greater heights, they coalesced almost instantly. Our study is regarding the former case.
Langmuir, 2010
Quantitative experimental data on the coalescence behavior of sessile droplets with different but completely miscible liquids are presented. The liquids consist of various aqueous mixtures of different nonvolatile diols and carbon acids with surface tensions ranging from 33 to 68 mN/m, contact angles between 9°and 20°, and viscosities from 1 to 12 cP. Two distinctly different coalescence behaviors, a delayed and a fast regime, are found. The transition between the two behaviors is remarkably sharp. It is found that the coalescence mode depends predominantly on the differences in the surface tensions of the two droplets. If the surface tension difference exceeds ∼3 mN/m, the coalescence is delayed. If it is less, droplet fusion occurs fast. Within the investigated parameter space, the transition seems independent from droplet size, absolute values of the surface tensions, and viscosity. Certain aspects of the experimental findings are explained with the simple hydrodynamic model presented in a recent publication.
Morphology and dynamics of droplet coalescence on a surface
Physical Review E, 2007
The coalescence of a pair of droplets on a surface is investigated experimentally with images from detailed flow visualisations revealing the morphology of the process. It is found that they merge and evolve to a final state with a footprint that is peanut like in shape, with bulges along the longer sides resulting from the effects of inertia during spreading. The associated dynamics involve a subtle interplay between ͑i͒ the motion of the wetting process due to relaxation of the contact angle and ͑ii͒ a rapid rise in free-surface height above the point where coalescence began due to negative pressure generated by curvature. During the early stages of the motion, a traveling wave propagates from the point of initial contact up the side of each droplet as liquid is drawn into the neck region, and only when it reaches the apex of each do their heights start to decrease. A further feature of the rapid rise in height of the neck region is that the free surface there overshoots significantly its final equilibrium position; it reaches a height greater than that of the starting droplets, producing a selfexcited oscillation that persists long after the system reaches its final morphological state in relation to its footprint.
Coalescence of sessile microdroplets subject to a wettability gradient on a solid surface
Physical Review E, 2016
While there are intensive studies on the coalescence of sessile macroscale droplets, there is little study on the coalescence of sessile microdroplets. In this paper, the coalescence process of two sessile microdroplets is studied by using a many-body dissipative particle dynamics numerical method. A comprehensive parametric study is conducted to investigate the effects on the coalescence process from the wettability gradient, hydrophilicity of the solid surface, and symmetric or asymmetric configurations. A water bridge is formed after two microdroplets contact. The temporal evolution of the coalescence process is characterized by the water bridge's radii parallel to the solid surface (W m) and perpendicular to the solid surface (H m). It is found that the changes of both H m and W m with time follow a power law; i.e., H m = β 1 τ β and W m = α 1 τ α. The growth of H m and W m depends on the hydrophilicity of the substrate. W m grows faster than H m on a hydrophilic surface, and H m grows faster than W m on a hydrophobic surface. This is due to the strong competition between capillary forces induced by the water-bridge curvature and the solid substrate hydrophobicity. Also, flow structure analysis shows that regardless of the coalescence type once the liquid bridge is formed the liquid flow direction inside the capillary bridge is to expand the bridge radius. Finally, we do not observe oscillation of the merged droplet during the coalescence process, possibly due to the significant effects of the viscous forces.
Investigation of drop coalescence via transient shape evolution: A sequential event based approach
arXiv: Fluid Dynamics, 2015
While a drop of liquid is placed on another liquid surface, two possible coalescence outcomes are observed. The parent drop bounces several times, floats and then disappears within the liquid pool without producing daughter droplets. This is called complete coalescence. Another outcome is the generation of secondary droplets from the primary drop itself. This is called partial coalescence. Repetitions of such phenomenon as a successive self-similar event is also known as coalescence cascade. In a nutshell, complete coalescence is governed strongly by swallowing mechanism whereas partial coalescence is attributed to slippage mechanism and solutal Marangoni flow. Here we use high speed camera and witness that water drop coalesces completely after impacting on pool of liquid whereas drop of non-ionic surfactant (TWEEN 20) coalesces partially. We also observe that number of daughter droplet generation is a strong function of surfactant concentration. Here we utilise the images to elabor...
The coalescence of water drops II. The coalescence problem and coalescence efficiency
Pure and Applied Geophysics PAGEOPH, 1977
By the use of the model of approaching drops the coalescence efficiencies of drops are computed. It is found that for interactions of drops at their terminal velocities the coalescence depends both on the size of the large drop and on the size ratio of the interacting drops in agreement with the experimental results of Whelpdale and List (1971) and .
Dynamics of Drop Coalescence on a Surface: The Role of Initial Conditions and Surface Properties
International Journal of Thermophysics, 2005
An investigation of the coalescence of two water drops on a surface is presented and compared with drop spreading. The associated capillary numbers are very low (< 10-5). The drops relax exponentially towards equilibrium. The typical relaxation time t c decreases with contact angle. t c is proportional to the drop size R, thus defining a characteristic velocity U* =R/t c. The corresponding U* values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = /, with the gasliquid surface tension and the viscosity). The dynamics of receding (coalescence) and spreading motion is found of the same order when coalescence or spreading is induced by a syringe. The dynamics of coalescence induced with the syringe deposition is systematically faster by an order of magnitude than condensation-induced coalescence. This disparity is explained by the coupling of the contact line motion with the oscillation of the drop observed for syringe deposition but absent for condensationinduced coalescence.
Industrial & Engineering Chemistry Fundamentals, 1971
The coalescence of a liquid drop at a planar interface was studied theoretically. The mechanism of coalescence was found to occur in two parts. In the first part (film thinning) a thin spherical shell of material between the drop and the interface is slowly squeezed out under the combined action of surface and gravity forces. When the film becomes sufficiently thin, the second part (film rupture) of the mechanism occurs. Because a denser liquid always overlies a less dense liquid at one of the interfaces of the film, the interface is inherently unstable with respect to long-wavelength disturbances (Taylor instability). If such a disturbance is introduced into the proper interface, the disturbance will grow exponentially in time until the film disintegrates, causing coalescence of the drop. A sufficiently intense disturbance of any wavelength can also rupture the film, causing coalescence. A maior source of disturbances is sonic noise. The effects of drop radius and the physical properties of the liquids are discussed.
Mechanism of Non-Coalescence for Liquid Droplets at the Air-Liquid Interface
This work addresses the physical origin and the conditions of generation of non-coalescence-droplets (N-C-D). Our results showed that there is a potential link between the N-C-D and an ink-jet printing defect, i.e. the satellite dots. Ink-jet printing is more widely used in the manufacturing of bioactive surfaces. Eliminating print defects will make the ink-jet technology a more economic and attractive option for the manufacturing of the bioactive surfaces.