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.

Effect of Superhydrophobicity on Impinging Droplet Heat Transfer

2010 ◽  
Author(s):  
Gary Rosengarten ◽  
Rita Tschaut
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
High Speed ◽  
Cold Water ◽  
Preliminary Investigation ◽  
Infrared Camera ◽  
Contact Angles ◽  
Superhydrophobic Surfaces ◽  
Droplet Shape

In this study we present a preliminary investigation into the effect of hydrophobicity on the heat transfer rate due to the impingement of cold water droplets on heated flat surfaces. Two extreme contact angles were compared; hydrophilic (∼20°) and superhydrophobic (∼160°) using different surface coatings on a thin metal substrates. Images of the droplet impingement were simultaneously recorded by a high speed camera and a high speed, high resolution infrared camera so we could correlate droplet shape and dynamics to the heat transfer rate. There is a large effect on both the droplet fluid dynamics and heat transfer between hydrophilic and superhydrophobic surfaces. The heat transfer difference between the superhydrophobic and hydrophilic cases is a complex interplay between the increased droplet contact line velocity due to induced slip and the insulating properties of the air gap. Overall we have shown significant reductions in both the instantaneous heat transfer rates and the overall cooling effect of droplets impinging on superhydrophobic surfaces relative to those for hydrophilic surfaces. In the range of droplet velocities varied (We = 50 to 190) there was little dependency of the heat transfer or fluid flow with impact velocity, due to the dominance of inertial forces.

Droplet Impacting on a Hydrophobic Surface: Influence of Surface Wetting State on Droplet Behavior

2020 ◽  
Vol 142 (7) ◽  
Author(s):  
Abba Abdulhamid Abubakar ◽  
Bekir Sami Yilbas ◽  
Ghassan Hassan ◽  
Hussain Al-Qahtani ◽  
Haider Ali ◽  
...  
Keyword(s):  
Contact Angle ◽  
Hydrophobic Surface ◽  
Transition Time ◽  
Restitution Coefficient ◽  
Surface Wetting ◽  
Local Pressure ◽  
Droplet Spreading ◽  
Droplet Dynamics ◽  
Droplet Shape ◽  
Droplet Behavior

Abstract Water droplet impacting onto a hydrophobic surface is considered and the influence of the surface wetting state on the droplet dynamics is examined. Pressure variation in the impacting droplet is predicted numerically using the level set model. The droplet spreading and the retraction on the hydrophobic surface are assessed for various wetting states of the hydrophobic surface. Experiment is carried out to validate the predictions of the droplet shape and the restitution coefficient. It is found that predictions of impacting droplet shape and the restitution coefficient agree with those obtained from the experiment. The local pressure peaks formed in the droplet fluid, particularly in the retraction period, causes alteration of the droplet vertical height and the shape. Droplet spreading is influenced by the wetting state of the hydrophobic surface; hence, increasing contact angle of the hydrophobic surface lowers the spreading diameter of the droplet on the surface. The transition time of the droplet changes with the wetting state of the hydrophobic surface such that increasing droplet contact angle reduces the transition time of the droplet on the surface. The droplet remains almost round after the first bounding for large contact angle hydrophobic surface.

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.

Studying of Contact Angle Friction and Contact Angle Hysteresis (CAH) Though Force Measurements

2012 ◽  
Author(s):  
Qi Ni ◽  
Timo Marschke ◽  
Samuel Steele ◽  
Najafi Seyed ◽  
Nathan B. Crane
Keyword(s):  
Contact Angle ◽  
Contact Line ◽  
Contact Angle Hysteresis ◽  
Contact Angles ◽  
Force Measurements ◽  
Droplet Motion ◽  
Characterization Methods ◽  
Receding Contact ◽  
Novel Method ◽  
Contact Line Friction

A novel method of measuring contact line friction and contact angle hysteresis is described. In this method, a droplet is constrained between two surfaces while the surface of interest initiates motion. The results are compared to conventional characterization methods such as measuring the angle of inclined plane for droplet motion and measuring advancing and receding contact angles by infusing/withdrawing liquid from the substrate. At slow speeds, the proposed method provides a measure of the hysteresis but can also capture information about the contact line friction and viscous affects. Droplet force dependence on droplet size (height/width) is also investigated.

Numerical Study of Droplet Motion in a Hydrophilic/Hydrophobic Microchannel

2008 ◽  
Author(s):  
Jiyoung Choi ◽  
Gihun Son
Keyword(s):  
Proton Exchange Membrane ◽  
Numerical Study ◽  
Side Wall ◽  
Contact Angles ◽  
Proton Exchange ◽  
Droplet Motion ◽  
Droplet Dynamics ◽  
Droplet Shape ◽  
Hydrophobic Microchannel ◽  
Exchange Membrane

The droplet dynamics in a hydrophilic/hydrophobic microchannel, which is applicable to a typical proton exchange membrane fuel cell (PEMFC), is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid-gas interface or droplet shape is determined by a level set method which is modified to treat the contact angles. The matching conditions at the interface are accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. Based on the numerical results, the droplet dynamics including the sliding and detachment of droplets is found to depend significantly on the contact angle. The effects of inlet flow velocity, droplet size and side wall on the droplet motion are quantified. Also, a droplet removal process is demonstrated on the combination of hydrophilic and hydrophobic surfaces.

Water–substrate physico-chemistry in wetting dynamics

2015 ◽  
Vol 781 ◽  
pp. 695-711 ◽  
Author(s):  
Petter Johansson ◽  
Andreas Carlson ◽  
Berk Hess
Keyword(s):  
Friction Factor ◽  
Contact Line ◽  
Continuum Model ◽  
Large Scale ◽  
Phase Field Model ◽  
Equilibrium Contact Angle ◽  
Wetting Front ◽  
Droplet Dynamics ◽  
Dynamics Simulations ◽  
Contact Line Friction

We consider the wetting of water droplets on substrates with different chemical composition and molecular spacing, but with an identical equilibrium contact angle. A combined approach of large-scale molecular dynamics simulations and a continuum phase field model allows us to identify and quantify the influence of the microscopic physics at the contact line on the macroscopic droplet dynamics. We show that the substrate physico-chemistry, in particular hydrogen bonding, can significantly alter the flow. Since the material parameters are systematically derived from the atomistic simulations, our continuum model has only one adjustable parameter, which appears as a friction factor at the contact line. The continuum model approaches the atomistic wetting rate only when we adjust this contact line friction factor. However, the flow appears to be qualitatively different when comparing the atomistic and continuum models, highlighting that non-trivial continuum effects can come into play near the interface of the wetting front.

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.

scholarly journals Wettability control of polymeric microstructures replicated from laser-patterned stamps

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Yangxi Fu ◽  
Marcos Soldera ◽  
Wei Wang ◽  
Stephan Milles ◽  
Kangfa Deng ◽  
...  
Keyword(s):  
Contact Angle Hysteresis ◽  
Contact Angles ◽  
Spatial Period ◽  
Structured Surfaces ◽  
High Adhesion ◽  
Direct Laser ◽  
Periodic Microstructures ◽  
Structure Height ◽  
Direct Laser Interference Patterning ◽  
Micropillar Arrays

AbstractIn this study, two-step approaches to fabricate periodic microstructures on polyethylene terephthalate (PET) and poly(methyl methacrylate) (PMMA) substrates are presented to control the wettability of polymeric surfaces. Micropillar arrays with periods between 1.6 and 4.6 µm are patterned by plate-to-plate hot embossing using chromium stamps structured by four-beam Direct Laser Interference Patterning (DLIP). By varying the laser parameters, the shape, spatial period, and structure height of the laser-induced topography on Cr stamps are controlled. After that, the wettability properties, namely the static, advancing/receding contact angles (CAs), and contact angle hysteresis were characterized on the patterned PET and PMMA surfaces. The results indicate that the micropillar arrays induced a hydrophobic state in both polymers with CAs up to 140° in the case of PET, without modifying the surface chemistry. However, the structured surfaces show high adhesion to water, as the droplets stick to the surfaces and do not roll down even upon turning the substrates upside down. To investigate the wetting state on the structured polymers, theoretical CAs predicted by Wenzel and Cassie-Baxter models for selected structured samples with different topographical characteristics are also calculated and compared with the experimental data.

scholarly journals Experimental study of the contact line friction coefficient

2020 ◽  
Vol 1677 ◽  
pp. 012156
Author(s):  
N Sibiryakov ◽  
W Zheng ◽  
O Kabov ◽  
B Bai
Keyword(s):  
Experimental Study ◽  
Friction Coefficient ◽  
Contact Line ◽  
Contact Line Friction

Modeling of Transport During Droplet Deposition and Spreading on Smooth and Microstructured Superhydrophilic Surfaces

2016 ◽  
Author(s):  
Jordan P. Mizerak ◽  
Van P. Carey
Keyword(s):  
Contact Angle ◽  
Contact Angles ◽  
Ansys Fluent ◽  
Effective Contact ◽  
Microstructured Surfaces ◽  
Static Contact ◽  
Droplet Behavior ◽  
Contact Line Pinning ◽  
The Impact ◽  
Superhydrophilic Surfaces

The dynamic behavior of impinging water droplets is studied in the context of varying surface morphologies on smooth and microstructured superhydrophilic surfaces. The goal of this study is to evaluate the capability of contact angle wall adhesion models to accurately produce spreading phenomena seen on a variety of surface types. We analyze macroscale droplet behavior, specifically spreading extent and impinging regime, in situations of varying microscale wetting character and surface morphology. Axisymmetric, volume of fluid (VOF) simulations with static contact angle wall adhesion are conducted in ANSYS Fluent. Simulations are performed on water for low Weber numbers (We<20) on surfaces with features of length scale 5–10μm. Advanced microstructured surfaces consisting of unique wetting characteristics and lengths on each face are also tested. Results show that while the contact angle wall adhesion model shows fair agreement for conventional surfaces, the model underestimates spreading by over 60% for surfaces exhibiting estimated contact angles below approximately 0.5°. Microstructured surfaces adapt the wetting behavior of smooth surfaces with higher effective contact angles based on contact line pinning on morphology features. The propensity of the model to produce Wenzel and Cassie-Baxter states is linked to the spreading radius, introducing an interdependency of microscale wetting and macroscale spreading behavior. Conclusions describing the impact of results on evaporative cooling are also discussed.

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