Contact line-based model for the Cassie-Wenzel transition of a sessile droplet on the hydrophobic micropillar-structured surfaces

The present study is motivated by interest in understanding of physical mechanisms that govern the effect of material and micro-structural characteristics of heat surface on boiling heat transfer and burnout at high heat fluxes. The effect was reported and investigated experimentally and analytically over several past decades. Only recently, with the advent of nanotechnology including microscale manufacturing, it becomes possible to perform high heat-flux boiling experiments with control of surface conditions. Of particular importance for practice is the potential for significant enhancement of boiling heat transfer (BHT) and critical heat flux (CHF) in pool and flow boiling on heaters with specially manufactured and controlled micro-structured surfaces. This enhancement is very important to a very wide range of engineering applications, like heat exchanger and cooling system, where maximum flux is needed. Currently, there are many controlled experiments that investigate such effect and they lend themselves a subject for detailed computational analysis. The focus of this study is micro-hydrodynamics of the evaporating thin liquid film at the receding triple contact line, corresponding to formation of dry spot in the footprint of a growing bubble. Parametric investigations are performed to assess the hypotheses that micro-structured surfaces enhance resilience to burnout due to residual liquid in the dry patch after contact line receding. Towards the study objective, a particle-based (mesh-less) method of computational fluid dynamics called Smoothed Particle Hydrodynamics (SPH) is adopted. The SPH method is selected for its capability to handle fluid dynamics in complex geometries and free surface problems without mass loss (characteristic of alternative interface capturing schemes used in mesh-based methods). Both surface tension and surface adhesion (hydrophilicity) are implemented and tested. The solid (heater) surface and manufactured micro-structures are represented by solid-type particles. Heat transfer, phase change (evaporation) and vapor dynamics are not included in the present simulation. The bouncing drop case measures the contact time of water droplet with solid surface. This case is used for “mesh” sensitivity (particle size) study and calibration of boundary conditions and surface tension coefficient. Subsequently, case studies are formulated and performed for contact line dynamics on heater surfaces with the fabricated Micro Pillar Arrays surfaces (MPA) and smooth surface. Variable characteristics include surface tension and pillar density on structured surface (modified by changing distance between pillars). First of all, residual fluid are found in all simulations with structured surface, while fluid are drained for smooth cases. For structured surface, it’s found that after the contact line recedes, fluid with higher surface tension resides in the dry patch more than fluid with lower coefficient, and the relation tends to be non-linear. While for smooth surface, all fluid will be drained after certain time and the relations are non-monotonic; it’s also found that the amount of residual fluid increase as the distance between pillars decreases until a limit. The fluid then starts to decrease with pillars being set further apart. The increase starts from 30 μm and the limit is around 10 μm.

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Uniform and Gaussian Ultraviolet Light Intensity Distribution on Droplet for Selective Area Deposition of Particles

Journal of Fluids Engineering ◽

10.1115/1.4047122 ◽

2020 ◽

Vol 142 (9) ◽

Author(s):

Tianyi Li ◽

Aravinda Kar ◽

Ranganathan Kumar

Keyword(s):

Contact Line ◽

Sessile Drop ◽

Uv Light ◽

Incident Light ◽

Droplet Surface ◽

Light Distribution ◽

Surface Tension Gradient ◽

Sessile Droplet ◽

Selective Area ◽

Light Profiles

Abstract Particle transport through Marangoni convection inside a sessile droplet can be controlled by the ultraviolet (UV) light distribution on the surface. The photosensitive solution changes the surface tension gradient on the droplet surface and can induce clockwise and counterclockwise circulations depending on the incident light distribution. In this paper, the stream function in the sessile drop has been evaluated in toroidal coordinates by solving the biharmonic equation. Multiple primary clockwise and counterclockwise circulations are observed in the droplet under various concentric UV light profiles. The downward dividing streamlines are expected to deposit the particles on the substrate, thus matching the number of deposited rings on the substrate with the number of UV light rings. Moffatt eddies appear near the contact line or centerline of the droplet depending on the UV light profile and its distance from the contact line.

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Molecular Origin of Fast Evaporation at the Solid–water–vapor Line in a Sessile Droplet

Nanoscale ◽

10.1039/d1nr07479b ◽

2022 ◽

Author(s):

Yongfeng Huang ◽

Cui Zhang ◽

Sheng Meng

Keyword(s):

Water Vapor ◽

Contact Line ◽

Water Molecules ◽

Sessile Droplet ◽

Vapor Line ◽

Molecular Origin ◽

Solid Water

By analyzing the behaviors of water molecules at the solid–water–vapor contact line, we explore the molecular origins of large evaporation rates at the contact line and find new ways to...

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Investigation of Sessile Droplet Wetting, Dynamics and Evaporation on Micro-Structured Substrates

Volume 1: Micro/Nanofluidics and Lab-on-a-Chip; Nanofluids; Micro/Nanoscale Interfacial Transport Phenomena; Micro/Nanoscale Boiling and Condensation Heat Transfer; Micro/Nanoscale Thermal Radiation; Micro/Nanoscale Energy Devices and Systems ◽

10.1115/mnhmt2016-6446 ◽

2016 ◽

Author(s):

Gui-Ping Zhu ◽

Kian-Soo Ong ◽

Karen Siew-Ling Chong ◽

Hu-Lin Huang ◽

Fei Duan

Keyword(s):

Droplet Size ◽

Contact Line ◽

Pure Water ◽

Volume Ratio ◽

Size Ratio ◽

Pure Liquid ◽

Nickel Surface ◽

Evaporation Process ◽

Sessile Droplet ◽

Evaporation Coefficient

The wetting, spreading and drying of pure liquid and nanofluid sessile droplets on a patterned solid surface were investigated systematically in terms of liquid and surface property. The patterned nickel surface was characterized with diamond, circular, hexagon and rectangular pillars. The size ratio between interval and pillars varies from 1.0 to 5.0. The study was firstly carried out for the effect of pure water droplet size on liquid spreading and droplet evaporation process on diamond-shape micro structured substrate with LInterval/LPillar=1.0. Larger amount of liquid leads to a larger wetting area. With fixed substrate (diamond, LInterval/LPillar=1.0) and droplet size (1 μm), mixture of DI water and Ethanol (volume ratio varies from 0.5 to 2.0) was used for generating droplets with different surface tension and evaporation coefficient. Fingering shape would generate on the contact line. With higher concentration of ethanol, the fingering effect is stronger and appeared in a shorter time. The contact area shrinks when increase the size ratio of interval and pillar. This would reduce the length of the contact line, and thus slow down the liquid evaporation. The role of pillar shape was examined based on time for complete evaporation. The effect of surface material on evaporation process was conducted on nickel and PMMA substrate fabricated with the same design. Additionally, investigations were conducted with solutions consisted with nanoparticles and DI water. The mixture were made at different weight ration to achieve concentration of nanoparticles varies from 0.02% to 0.18% with an interval at 0.04%.

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Dynamics of sessile drops. Part 1. Inviscid theory

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.582 ◽

2014 ◽

Vol 760 ◽

pp. 5-38 ◽

Cited By ~ 29

Author(s):

J. B. Bostwick ◽

P. H. Steen

Keyword(s):

Contact Angle ◽

Contact Line ◽

Dynamic Contact ◽

Linear Operators ◽

Sessile Droplet ◽

Physical Reason ◽

Spherical Cap ◽

Static Contact ◽

Dynamic Contact Line ◽

Line Condition

AbstractA sessile droplet partially wets a planar solid support. We study the linear stability of this spherical-cap base state to disturbances whose three-phase contact line is (i) pinned, (ii) moves with fixed contact angle and (iii) moves with a contact angle that is a smooth function of the contact-line speed. The governing hydrodynamic equations for inviscid motions are reduced to a functional eigenvalue problem on linear operators, which are parameterized by the base-state volume through the static contact angle and contact-line mobility via a spreading parameter. A solution is facilitated using inverse operators for disturbances (i) and (ii) to report frequencies and modal shapes identified by a polar $k$ and azimuthal $l$ wavenumber. For the dynamic contact-line condition (iii), we show that the disturbance energy balance takes the form of a damped-harmonic oscillator with ‘Davis dissipation’ that encompasses the dynamic effects associated with (iii). The effect of the contact-line motion on the dissipation mechanism is illustrated. We report an instability of the super-hemispherical base states with mobile contact lines (ii) that correlates with horizontal motion of the centre-of-mass, called the ‘walking’ instability. Davis dissipation from the dynamic contact-line condition (iii) can suppress the instability. The remainder of the spectrum exhibits oscillatory behaviour. For the hemispherical base state with mobile contact line (ii), the spectrum is degenerate with respect to the azimuthal wavenumber. We show that varying either the base-state volume or contact-line mobility lifts this degeneracy. For most values of these symmetry-breaking parameters, a certain spectral ordering of frequencies is maintained. However, because certain modes are more strongly influenced by the support than others, there are instances of additional modal degeneracies. We explain the physical reason for these and show how to locate them.

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Influence of substrate temperature on Marangoni convection instabilities in a sessile droplet evaporating at constant contact line mode

International Journal of Heat and Mass Transfer ◽

10.1016/j.ijheatmasstransfer.2018.11.155 ◽

2019 ◽

Vol 131 ◽

pp. 1270-1278 ◽

Cited By ~ 12

Author(s):

Tian-Shi Wang ◽

Wan-Yuan Shi

Keyword(s):

Substrate Temperature ◽

Contact Line ◽

Marangoni Convection ◽

Sessile Droplet ◽

Influence Of Substrate

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Pinning mechanism of advancing sessile droplet on superhydrophobic surfaces

RSC Advances ◽

10.1039/c4ra06428c ◽

2014 ◽

Vol 4 (67) ◽

pp. 35649-35652 ◽

Cited By ~ 4

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.

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

Colloids and Interfaces ◽

10.3390/colloids5010008 ◽

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.

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