Pinning mechanism of advancing sessile droplet on superhydrophobic surfaces

RSC Advances ◽  
2014 ◽  
Vol 4 (67) ◽  
pp. 35649-35652 ◽  
Author(s):  
Jun Wu ◽  
Jun Xia ◽  
Wei Lei ◽  
Bao-ping Wang
Keyword(s):  
Contact Line ◽  
Superhydrophobic Surface ◽  
Evolutionary Process ◽  
Contact Angles ◽  
Superhydrophobic Surfaces ◽  
Droplet Volume ◽  
Phase Contact ◽  
Sessile Droplet ◽  
Pinning Mechanism ◽  
Stage D

The evolution of the “local triple-phase contact line” with increasing droplet volume on a micropillared superhydrophobic surface, from (a) the initial contacting stage to (b) the pinning stage to (c) the depinning stage. (d) The sketch of the evolutionary process of local contact angles.

scholarly journals Dependency of Contact Angles on Three-Phase Contact Line: A Review

2021 ◽  
Vol 5 (1) ◽  
pp. 8
Author(s):  
H. Yildirim Erbil
Keyword(s):  
Contact Angle ◽  
Contact Line ◽  
Contact Angle Hysteresis ◽  
Contact Angles ◽  
Contact Radius ◽  
Phase Contact ◽  
Sessile Droplet ◽  
Line Energy ◽  
Three Phase ◽  
Tension Term

The wetted area of a sessile droplet on a practical substrate is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, contact angles of droplets have been studied for more than two hundred years, there are still some unanswered questions. In the last two decades, it was experimentally proven that the advancing and receding contact angles, and the contact angle hysteresis of rough and chemically heterogeneous surfaces, are determined by interactions of the liquid and the solid at the three-phase contact line alone, and the interfacial area within the contact perimeter is irrelevant. However, confusion and misunderstanding still exist in this field regarding the relationship between contact angle and surface roughness and chemical heterogeneity. An extensive review was published on the debate for the dependence of apparent contact angles on drop contact area or the three-phase contact line in 2014. Following this old review, several new articles were published on the same subject. This article presents a review of the novel articles (mostly published after 2014 to present) on the dependency of contact angles on the three-phase contact line, after a short summary is given for this long-lasting debate. Recently, some improvements have been made; for example, a relationship of the apparent contact angle with the properties of the three-phase line was obtained by replacing the solid–vapor interfacial tension term, γSV, with a string tension term containing the edge energy, γSLV, and curvature of the triple contact line, km, terms. In addition, a novel Gibbsian thermodynamics composite system was developed for a liquid drop resting on a heterogeneous multiphase and also on a homogeneous rough solid substrate at equilibrium conditions, and this approach led to the same conclusions given above. Moreover, some publications on the line energy concept along the three-phase contact line, and on the “modified” Cassie equations were also examined in this review.

Direct Numerical Simulation of the Microscale Fluid Flow and Heat Transfer in the Three-Phase Contact Line Region During Evaporation

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Stefan Batzdorf ◽  
Tatiana Gambaryan-Roisman ◽  
Peter Stephan
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Contact Line ◽  
Thin Liquid Film ◽  
Contact Angles ◽  
Length Scale ◽  
Phase Contact ◽  
Flow And Heat Transfer ◽  
Three Phase

The heat and mass transfer close to the apparent three-phase contact line is of tremendous importance in many evaporation processes. Despite the extremely small dimensions of this region referred to as the microregion compared to the macroscopic length scale of a boiling process, a considerable fraction of heat can be transferred in this region. Due to its small characteristic length scale, physical phenomena are relevant in the microregion, which are completely negligible on the macroscopic scale, including the action of adhesion forces and the interfacial heat resistance. In the past, models have been developed taking these effects into account. However, so far these models are based on the assumption of one-dimensional (1D) heat conduction, and the flow within the thin liquid film forming the microregion near the apparent three-phase contact line is modeled utilizing the lubrication approximation. Hence, the application of existing models is restricted to small apparent contact angles. Moreover, the effects of surface structures or roughness are not included in these lubrication models. To overcome these limitations, a direct numerical simulation (DNS) of the liquid flow and heat transfer within the microregion is presented in this paper. The DNS is employed for validation of the existing lubrication model and for investigation of the influence of surface nanostructures on the apparent contact angle and in particular on the heat transfer within the microregion.

Effect of Three-Phase Contact Line Topology on Dynamic Contact Angles on Heterogeneous Surfaces

Langmuir ◽  
2007 ◽  
Vol 23 (23) ◽  
pp. 11673-11676 ◽  
Author(s):  
Neeharika Anantharaju ◽  
Mahesh V. Panchagnula ◽  
Srikanth Vedantam ◽  
Sudhakar Neti ◽  
Svetlana Tatic-Lucic
Keyword(s):  
Contact Line ◽  
Dynamic Contact ◽  
Contact Angles ◽  
Heterogeneous Surfaces ◽  
Phase Contact ◽  
Three Phase

Effects of Interfacial Position on Drag Reduction in a Superhydrophobic Microchannel

2008 ◽  
Author(s):  
Ryan Enright ◽  
Tara Dalton ◽  
Tom N. Krupenkin ◽  
Paul Kolodner ◽  
Marc Hodes ◽  
...  
Keyword(s):  
Pressure Drop ◽  
Drag Reduction ◽  
Flow Rate ◽  
Contact Line ◽  
Superhydrophobic Surface ◽  
Parallel Plate ◽  
Laser Scanning ◽  
Liquid Interface ◽  
Experimental Results ◽  
Superhydrophobic Surfaces

The use of superhydrophobic surfaces in confined flows is of particular interest as these surfaces have been shown to exhibit a drag reduction effect that is orders of magnitude larger than those due to molecular slip. In this paper we present experimental results of the pressure-driven flow of water in a parallel-plate microchannel having a no-slip upper wall and a superhydrophobic lower wall. Pressure-drop versus flow-rate measurements characterize the apparent slip behavior of the superhydrophobic surfaces with varying pillar-to-pillar pitch spacing and pillar diameter. The superhydrophobic surface consists of a square array of cylindrical pillars that are fabricated by deep reactive ion etching on silicon and coated with a hydrophobic fluoropolymer. A major challenge, in correlating our experimental results with existing theoretical predictions, is uncertainty in the location of the gas/liquid interface and the associated gas/liquid/solid contact line within the pillar features comprising the superhydrophobic surface. We present experimental results, from laser-scanning confocal microscopy, that measure the location of the gas-liquid interface and associated contact line for fluid flowing through a parallel-plate microchannel. Knowledge of the contact line location is then used to correlate experimental pressure-drop versus flow-rate data with a theoretical model based on porous-flow theory that takes into account partial penetration of liquid into a superhydrophobic surface.

scholarly journals Sessile Droplets on Deformable Substrates

2018 ◽  
Vol 2 (4) ◽  
pp. 56
Author(s):  
Gulraiz Ahmed ◽  
Nektaria Koursari ◽  
Anna Trybala ◽  
Victor M. Starov
Keyword(s):  
Free Energy ◽  
Contact Line ◽  
Liquid Droplet ◽  
Excess Free Energy ◽  
Surface Elasticity ◽  
Contact Angles ◽  
Interfacial Behavior ◽  
Phase Contact ◽  
Technological Advances ◽  
Three Phase

Wetting of deformable substrates has gained significant interest over the past decade due to a multiplicity of industrial and biological applications. Technological advances in the area of interfacial science have given rise to the ability to capture interfacial behavior between a liquid droplet and an elastic substrate. Researchers have developed several theories to explain the interaction between the two phases and describe the process of wetting of deformable/soft substrates. A summary of the most recent advances on static wetting of deformable substrates is given in this review. It is demonstrated that action of surface forces (disjoining/conjoining pressure) near the apparent three-phase contact line should be considered. Any consideration of equilibrium droplets on deformable (as well as on non-deformable) substrates should be based on consideration of the excess free energy of the system. The equilibrium shapes of both droplet and deformable substrate should correspond to the minimum of the excess free energy of the system. It has never been considered in the literature that the obtained equilibrium profiles must satisfy sufficient Jacobi’s condition. If Jacobi’s condition is not satisfied, it is impossible to claim that the obtained solution really corresponds to equilibrium. In recently published studies, equilibrium of droplets on deformable substrates: (1) provided a solution that corresponds to the minimum of the excess free energy; and (2) the obtained solution satisfies the Jacobi’s condition. Based on consideration of disjoining/conjoining pressure acting in the vicinity of the apparent three-phase contact line, the hysteresis of contact angle of sessile droplets on deformable substrates is considered. It is shown that both advancing and receding contact angles decrease as the elasticity of the substrate is increased and the effect of disjoining/conjoining pressure is discussed. Fluid inside the droplet partially wets the deformable substrate. It is shown that just these forces coupled with the surface elasticity determine the deformation of the deformable substrates.

Drop spreading on a superhydrophobic surface: pinned contact line and bending liquid surface

2017 ◽  
Vol 19 (22) ◽  
pp. 14442-14452 ◽  
Author(s):  
Yanbin Wang ◽  
Joseph Eugene Andrews ◽  
Liangbing Hu ◽  
Siddhartha Das
Keyword(s):  
Contact Line ◽  
Superhydrophobic Surface ◽  
Liquid Surface ◽  
Liquid Interface ◽  
Phase Contact ◽  
Drop Spreading ◽  
Three Phase ◽  
Air Liquid Interface

On a superhydrophobic surface, a drop spreads by the bending of the air–liquid interface with the three-phase contact line remaining pinned.

Contact Angle Measurement Through Atomic Force Microscopy

2011 ◽  
Author(s):  
Jiapeng Yu ◽  
Hao Wang
Keyword(s):  
Thin Film ◽  
Atomic Force Microscopy ◽  
Contact Line ◽  
Contact Angles ◽  
Angle Measurement ◽  
Optical Methods ◽  
Phase Contact ◽  
Force Microscopy ◽  
Atomic Force ◽  
Three Phase

Understanding the structure near the three-phase contact line is critical for a comprehensive understanding of the thin-film region when a liquid partially wets a planer substrate. Despite numerous theoretical and simulation efforts found literature, an accurate experiment is difficult to conduct because of how small its scale. In the present work the accurate geometry of the region near the three-phase contact line was obtained by directly scanning the thin-film region with atomic force microscopy (AFM). The contact angles were directly extracted from the results and compared with the ones measured from traditional optical methods.

Droplet Shapes on Superhydrophobic Surfaces Under Electrowetting Actuation

2011 ◽  
Author(s):  
S. Ravi Annapragada ◽  
Jayathi Y. Murthy ◽  
Suresh V. Garimella
Keyword(s):  
Contact Line ◽  
Intermediate Energy ◽  
Contact Angles ◽  
Superhydrophobic Surfaces ◽  
Droplet Dynamics ◽  
Droplet Shape ◽  
Structured Surfaces ◽  
Droplet Behavior ◽  
Wetting Process ◽  
Contact Line Friction

Droplet behavior on structured surfaces has recently generated a lot of interest due to its application to self-cleaning surfaces and in microfluidic devices. In this paper, the droplet shape and the droplet state on superhydrophobic surfaces are predicted using the Volume of Fluid (VOF) approach. Various structured surfaces are considered and the apparent contact angles are extracted from the predicted droplet shapes. Droplet dynamics under electrowetting are also modeled, including contact line friction. The model is validated against in-house experiments and experiments from the literature. The droplet state, droplet shape and apparent contact angles match well with the experimental measurements. The Cassie and Wenzel states on structured surfaces are also accurately predicted. Further, the electrowetting-induced transition from the Cassie to the Wenzel state and the reversal to the Cassie state is predicted for two different superhydrophobic surfaces. The transient wetting process, intermediate energy states and droplet shapes during electrowetting are simulated. The effective contact line friction coefficient on pillared surfaces is predicted to be 0.14 Ns/m2, consistent with published values.

Forces Acting on Sessile Droplet on Inclined Surfaces

2009 ◽  
Author(s):  
S. Ravi Annapragada ◽  
Jayathi Y. Murthy ◽  
Suresh V. Garimella
Keyword(s):  
Contact Angle ◽  
Droplet Size ◽  
Contact Line ◽  
Contact Area ◽  
Contact Angles ◽  
Force Model ◽  
Sessile Droplet ◽  
Droplet Motion ◽  
Droplet Shape ◽  
Receding Contact

Although many analytical, experimental and numerical studies have focused on droplet motion, the mechanics of the droplet while still in its static state, and just before motion starts, are not well understood. A study of static droplets would shed light on the threshold voltage (or critical inclination) for initiating electrically (or gravitationally) induced droplet motion. Before the droplet starts to move, the droplet shape changes such that the forces acting at the triple contact line balance the actuation forces. These contact line forces are governed by the contact angles along the contact line. The contact angle varies from an advancing angle at the leading edge to a receding angle at the trailing edge of the droplet. The present study seeks to understand and predict these forces at the triple contact line. The droplet shape, as well as the advancing and receding contact angles, is experimentally measured as a function of droplet size under the action of a gravitational force at different inclination angles. The advancing and receding contact angles are correlated with static contact angle and Bond number. A Volume of Fluid - Continuous Surface Force model with varying contact angles along the triple contact line is developed to predict the same. The model is first verified against a two-dimensional analytical solution. It is then used to simulate the shape of a sessile droplet on an incline at various angles of inclination and to determine the critical angle of inclination as a function of droplet size. Good agreement is found between experimental measurements and predictions. The contact line profile and contact area are also predicted. The contact area predictions based on a spherical-cap assumption are also compared against the numerical predictions.

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