Sliding variability of droplets on a hydrophobic incline due to surface entrained air bubbles

Ling, William Yeong Liang; Ng, Tuck Wah; Neild, Adrian; Zheng, Quanshui

doi:10.1016/j.jcis.2010.11.027

Cited by 23 publications

(14 citation statements)

References 38 publications

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“…This is aided to some extent by the ability of AL but not RL to breach initially, similar to situations that were reported previously. 14−16 With evaporation, this contact angle disparity reduces and AL gradually restores back toward its condition before breaching. From the quantitative plots of advancing and receding angles with time (Figure 13A), it can be seen that the rate with which the advancing contact angle reduces is somewhat similar to the rate at which the receding contact angle increases.…”

Section: Resultsmentioning

confidence: 99%

“…It has been shown that with onset of tilt the advancing contact line breaches first, leaving the receding contact line to determine the drop’s ability to detach. 14−16 This then allows the ability of the drop to resist movement to be enhanced 17−19 or for the shape to be altered 20,21 (if the surface is hydrophilic) by providing additional pinning modes at the locale of the receding contact line.…”

Section: Introductionmentioning

confidence: 99%

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Drops on a Superhydrophobic Hole Hanging On under Evaporation

Chung¹,

Huynh²,

Katariya³

et al. 2017

ACS Omega

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Drops with larger volumes placed over a superhydrophobic (SH) surface with a hole do not fall through unless they are evaporated to a size that is small enough. This feature offers the ability to preconcentrate samples for biochemical analysis. In this work, the influence of pinning on the behavior of drops placed on a 0.1 mm thick SH substrate with a 2 mm diameter hole as they evaporated was investigated. With 16 μL of water dispensed, the sessile drop component volume was initially higher than that of the overhanging drop component and maintained this until the later stages where almost identical shapes were attained and full evaporation was achieved without falling off the hole. With 15 μL of water dispensed, the volume of the sessile drop was initially higher than that of the overhanging drop component but the liquid body was able to squeeze through the hole after 180 s due to the contact line not having sufficient pinning strength when it encountered the edge of the hole. This resulted in the liquid body either falling through the hole or remaining pinned with an oval-like shape. When it did not fall-off, the liquid body had volume and contact angle characteristics for the sessile drop and overhanging drop components that were reversed. In the later stages, however, nearly identical shapes were again attained and full evaporation was achieved without falling off the hole. The effects of pinning, despite the substrate being SH, offer another path toward achieving practical outcomes with liquid bodies without the need for chemical surface functionalization. Similarities and differences could be seen in the behavior of a sessile drop on a SH plate that was inclined at 30° to the horizontal and evaporated.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Drops on a Superhydrophobic Hole Hanging On under Evaporation

Chung¹,

Huynh²,

Katariya³

et al. 2017

ACS Omega

Self Cite

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“…Discrepancies from this expression of force balance are far from rare as a result of different theoretical assumptions or different conditions for the experiments, 75−78 whereas in this form it cannot explicitly account for the contributions of the capillary bridges developed in textured surfaces. 51 A different approach in treating this problem is provided by assuming the in-plane retention forces 75,79 or the vertical pinning forces at the receding contact line 78 for each capillary bridge. In this latter approach, the vertical pinning force for each (j) capillary bridge is given by where P j is the perimeter of an individual surface feature and θ r j is the receding angle local to the jth feature.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…Even though this method has been proven to be rather versatile in delivering mobility manipulation, as described above, we have not yet provided the underlying working mechanism. The application of the backpressure introduces a variety of changes 51 in wetting-related parameters, such as in the advancing and mainly in the receding contact angle and in the liquid−solid contact area. Additional Laplace forces appear, which are asymmetric due to the inclination and deform the droplet shape.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Moreover, it has been proven that the force exerted by each bubble alone cannot account for the downward droplet movement. 51 In this work, we shed light on the underlying working mechanisms of droplet actuation and movement induced by backpressure application through a porous medium. The problem is followed by means of simulations encompassing the momentum conservation and the continuity equations along with the Cahn−Hilliard phase-field equations in a 2D domain and by employing the governing vertical force balance accounting for the partial depinning of the receding contact line.…”

Section: ■ Introductionmentioning

confidence: 99%

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Droplet Mobility Manipulation on Porous Media Using Backpressure

Vourdas¹,

Pashos

Kokkoris

et al. 2016

Langmuir

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Wetting phenomena on hydrophobic surfaces are strongly related to the volume and pressure of gas pockets residing at the solid-liquid interface. In this study, we explore the underlying mechanisms of droplet actuation and mobility manipulation when backpressure is applied through a porous medium under a sessile pinned droplet. Reversible transitions between the initially sticky state and the slippery states are thus incited by modulating the backpressure. The sliding angles of deionized (DI) water and ethanol in DI water droplets of various volumes are presented to quantify the effect of the backpressure on the droplet mobility. For a 50 μL water droplet, the sliding angle decreases from 45 to 0° when the backpressure increases to ca. 0.60 bar. Significantly smaller backpressure levels are required for lower surface energy liquids. We shed light on the droplet actuation and movement mechanisms by means of simulations encompassing the momentum conservation and the continuity equations along with the Cahn-Hilliard phase-field equations in a 2D computational domain. The droplet actuation mechanism entails depinning of the receding contact line and movement by means of forward wave propagation reaching the front of the droplet. Eventually, the droplet skips forward. The contact line depinning is also corroborated by analytical calculations based on the governing vertical force balance, properly modified to incorporate the effect of the backpressure.

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