The effect of contact angle hysteresis on droplet coalescence and mixing

Nilsson, Michael; Rothstein, Jonathan P.

doi:10.1016/j.jcis.2011.07.086

Cited by 64 publications

(42 citation statements)

References 48 publications

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“…These directional SH textures exhibit anisotropic wetting behavior [30][31][32][33] and generate a contact angle hysteresis, 34 which has already been employed for sorting droplets in a droplet-based microfluidic device, 35,36 and should be important for understanding a motion of drops on inclined anisotropic surfaces. 37 It is natural to suggest that on materials with a high density of anisotropic patterns, the contact angle hysteresis can be relatively large and anisotropic owing to an enhanced solid/liquid contact (compared to dilute micropillars).…”

Section: Introductionmentioning

confidence: 99%

Contact angle hysteresis on superhydrophobic stripes

Dubov

Mourran

Möller

et al. 2014

The Journal of Chemical Physics

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We study experimentally and discuss quantitatively the contact angle hysteresis on striped superhydrophobic surfaces as a function of a solid fraction, ϕS. It is shown that the receding regime is determined by a longitudinal sliding motion of the deformed contact line. Despite an anisotropy of the texture the receding contact angle remains isotropic, i.e., is practically the same in the longitudinal and transverse directions. The cosine of the receding angle grows nonlinearly with ϕS. To interpret this we develop a theoretical model, which shows that the value of the receding angle depends both on weak defects at smooth solid areas and on the strong defects due to the elastic energy of the deformed contact line, which scales as ϕS(2)lnϕS. The advancing contact angle was found to be anisotropic, except in a dilute regime, and its value is shown to be determined by the rolling motion of the drop. The cosine of the longitudinal advancing angle depends linearly on ϕS, but a satisfactory fit to the data can only be provided if we generalize the Cassie equation to account for weak defects. The cosine of the transverse advancing angle is much smaller and is maximized at ϕS ≃ 0.5. An explanation of its value can be obtained if we invoke an additional energy due to strong defects in this direction, which is shown to be caused by the adhesion of the drop on solid sectors and is proportional to ϕS(2). Finally, the contact angle hysteresis is found to be quite large and generally anisotropic, but it becomes isotropic when ϕS ≤ 0.2.

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Section: Introductionmentioning

confidence: 99%

Contact angle hysteresis on superhydrophobic stripes

Dubov

Mourran

Möller

et al. 2014

The Journal of Chemical Physics

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“…These striped textures amplify electrokinetic pumping 22,23 and are employed for sorting droplets. [24][25][26] We study a wetting transition by monitoring the evaporation of a drop. An evaporating drop is known to produce various intriguing phenomena, such as coffee stains 27 or wine tears, 28 and has been already studied with isotropic SH surfaces, 7,29,30 although the quantitative understanding is still at its infancy.…”

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confidence: 99%

Regimes of wetting transitions on superhydrophobic textures conditioned by energy of receding contact lines

Dubov

Mourran

Möller

et al. 2015

Applied Physics Letters

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We discuss an evaporation-induced wetting transition on superhydrophobic stripes, and show that depending on the elastic energy of the deformed contact line, which determines the value of an instantaneous apparent contact angle, two different scenarios occur. For relatively dilute stripes the receding angle is above 90 • , and the sudden impalement transition happens due to an increase of a curvature of an evaporating drop. For dense stripes the slow impregnation transition commences when the apparent angle reaches 90 • and represents the impregnation of the grooves from the triple contact line towards the drop center.

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“…6,[9][10][11] In the slow process, contact lines move and the drop relaxes to a circular shape over longer time scales, slowed by the dissipation at the moving contact lines and fluid flow. 9,10,12 Experimentally, the initial bridge healing of sessile drops has been probed using high speed video microscopy of fluids including mercury, 13 silicone oils, 7 water, [14][15][16][17] and organic fluids. 11,[18][19][20] Different mechanisms have been used to cause the drops to touch and coalesce.…”

Section: -2 Zhang Et Almentioning

confidence: 99%

“…11,[18][19][20] Different mechanisms have been used to cause the drops to touch and coalesce. Drops are allowed to spread spontaneously into one another, 7,19 one drop is deposited onto another, 13,18,20 or one drop is blown into another 17 on a surface until they meet and merge. In other cases, drops are forced to grow into one another due to an external supply of liquid 14,15 or condensing vapor.…”

Section: -2 Zhang Et Almentioning

confidence: 99%

Gravity driven current during the coalescence of two sessile drops

et al. 2015

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Coalescence of liquid drops is critical in many phenomena such as emulsion stability, inkjet printing, and coating applications. For sessile drops on a solid surface, the coalescence process is more complicated than the coalescence of drops suspended in a fluid medium as a result of the coupling of the contact line motions to the fluid flow. In this paper, we use video microscopy to track the evolution of the interfaces and contact lines as well as the internal fluid motion within a merged sessile droplet. In this study, the fluids in the coalescing drops are miscible and have similar surface tensions and drop volumes but different viscosities and densities. Coalescence occurs in three stages. During the first stage, rapid healing of the bridge between the drops occurs just after they touch. In the second stage, slower rearrangement of the liquids occurs. We show that these intermediate rearrangements are driven by gravity even for density differences of the two fluids as small as 1%. For the systems examined, little to no mixing occurs during these first two stages. Finally, in the third stage, diffusion leads to mixing of the fluids. Dimensional analysis reveals the scaling of the intermediate flow behavior as a function of density difference and geometric dimensions of the merged drop; however, the scaling with viscosity is more complicated, motivating development of a lubrication analysis of the coalescence problem. Numerical calculations based on the lubrication analysis capture aspects of the experimental observations and reveal the governing forces and time scales of the coalescence process. The results reveal that internal fluid motions persist over much longer time scales than imaging of the external interface alone would reveal. Furthermore, nearly imperceptible motions of the external composite drop interface can lead to important deviations from the predominant gravity current scaling, where viscous resistance of the lighter fluid layer plays a significant role in the internal fluid motion. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907725] INTRODUCTIONCoalescence of drops is critically important to numerous technological and natural processes. For example, coalescence of drops on surfaces leads to the formation of a uniform film in spray coating and influences heat transfer in spray cooling. [1][2][3] While the coalescence of drops suspended in a fluid medium has been examined extensively, 4,5 coalescence of two sessile drops on a solid surface adds the feature that the contact lines of the drops must move during the merging process. Coalescence of sessile drops of identical fluids shows a rapid bridge healing process and a slow contact line relaxation. In the rapid process, which is controlled by the interplay of inertia, viscosity, and capillarity, 7,8 the drops touch and a bridge rapidly forms and heals while the contact line does not move appreciably. 6,[9][10][11] In the slow process, contact lines move and the drop relaxes to a circular shape over longer time scales, slowed by th...

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The effect of contact angle hysteresis on droplet coalescence and mixing

Cited by 64 publications

References 48 publications

Contact angle hysteresis on superhydrophobic stripes

Contact angle hysteresis on superhydrophobic stripes

Regimes of wetting transitions on superhydrophobic textures conditioned by energy of receding contact lines

Gravity driven current during the coalescence of two sessile drops

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