Coalescence of sessile microdroplets subject to a wettability gradient on a solid surface

Abstract

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.

Effect of Wetting and Dewetting Dynamics on Atomic Force Microscopy Measurements

“…Because MDPD is a coarse-grained particle simulation method, reduced units are commonly used to link to physical quantities. The majority of MDPD studies use dimensionless mass, distance, and time units that are not rigorously tied to physical scales, therefore restricting the scope to only qualitative insights. − Only recently, attempts were made to map MDPD scales to physical scales by choosing various MDPD parameters, such as cutoff radius. , …”

On Droplet Coalescence in Quasi-Two-Dimensional Fluids

“…Dynamics of floating objects and their interactions with the flow in restricted geometries are of paramount interest in physical, chemical, and biological systems. − Furthermore, coalescence and merging of liquid objects such as liquid droplets have a large practical relevance. ,− One can find coalescence of droplets in everyday life such as during rain drop formation, in colloidal emulsions like mayonnaise and creams, , and in the merging of powders into a homogeneous material by heating (sintering). Industrial applications, such as ink-jet printing, coating processes and emulsion stability, oil recovery, etc., require a better and detailed understanding of the merging dynamics of liquid droplets.…”

“…The coalescence of 3D droplets exhibits three different dynamic regimes: the inertial limited viscous regime at the very early stage of the neck formation, the viscous regime dominating at the sufficiently early time of the neck formation, the inertial regime occurring at later times. ,, Interactions with solid and viscoelastic substrates strongly affect the dynamics of the coalescing droplets. ,,− …”

On droplets coalescence in a quasi-2D fluid

“…Dynamics of floating objects and their interactions with the flow in restricted geometries is of paramount interest in physical, chemical and biological systems [1][2][3] . Furthermore, the coalescence and merging of liquid objects such as liquid droplets has a large practical relevance 2,[4][5][6][7][8][9] . One can find coalescence of droplets in everyday life such as during rain drop formation 10 , it occurs in colloidal emulsions like mayonnaise and creams 11,12 , one can also observe the merging of powders into a homogeneous material by heating (sintering).…”

“…The coalescence of 3D droplets exhibits of three different dynamic regimes such as the inertial limited viscous regime at the very early stage of the neck formation, viscous regime dominating at the sufficiently early time of the neck formation, the inertial regime occurring at later times 19,23,27 . Interactions with solid and viscoelastic substrate strongly affect the dynamics of the coalescing droplets 5,18,[28][29][30] .…”