Meniscus and viscous forces during separation of hydrophilic and hydrophobic smooth/rough surfaces with symmetric and asymmetric contact angles

2008 ◽  
Vol 366 (1870) ◽  
pp. 1627-1647 ◽  
Author(s):  
Shaobiao Cai ◽  
Bharat Bhushan
Keyword(s):  
Contact Angle ◽  
Hydrophobic Surface ◽  
Separation Distance ◽  
Contact Angles ◽  
Viscous Force ◽  
Meniscus Force ◽  
Meniscus Height ◽  
Viscous Forces ◽  
Separation Time ◽  
Asymmetric Contact

Adhesive or repulsive forces contributed by both meniscus and viscous forces can be significant and become one of the main reliability issues when the contacting surfaces are ultra smooth, and the normal load is small, as is common for micro/nano devices. In this study, both meniscus and viscous forces during separation for smooth and rough hydrophilic and hydrophobic surfaces are studied. The effects of separation distance, initial meniscus height, separation time, contact angle and roughness are presented. Meniscus force decreases with an increase of separation distance, whereas the viscous force has an opposite trend. Both forces decrease with an increase of initial meniscus height. An increase of separation time, initial meniscus height or a decrease of contact angle leads to an increase of critical meniscus area at which both forces are equivalent. An increase in contact angle leads to a decrease of attractive meniscus force but an increase of repulsive meniscus force (attractive or repulsive dependent on hydrophilic or hydrophobic surface, respectively). Contact angle has a limited effect on the viscous force. For asymmetric contact angles, the magnitude of the meniscus force and the critical meniscus area are in between the values for the two angles. An increase in the number of surface asperities (roughness) leads to an increase of meniscus force; however, its effect on viscous force is trivial. A slightly attractive force is observed for the hydrophobic surface during the end stage of separation though the magnitude is small. The study provides a fundamental understanding of the physics of the separation process and it can be useful for control of the forces in nanotechnology applications.

Theoretical Study on the Dynamic Separation of Two Microplates with an Intervening Liquid Meniscus

2011 ◽  
Vol 148-149 ◽  
pp. 731-735
Author(s):  
Le Feng Wang ◽  
Wei Bin Rong ◽  
Bin Guo ◽  
Bing Shao
Keyword(s):  
Theoretical Study ◽  
Separation Distance ◽  
Contact Angles ◽  
Viscous Force ◽  
Parallel Plates ◽  
Liquid Meniscus ◽  
Meniscus Force ◽  
Separation Time ◽  
Simulation Parameters ◽  
Sudden Jump

The dynamic separation of two parallel microplates with an intervening liquid meniscus is studied theoretically. The dynamic model considering the meniscus force and the viscous force between the plates in the separation process was developed. With typical simulation parameters, the magnitudes of various forces were compared before the separation of two parallel plates occurs. It was found that the meniscus force decreases gradually with the increase of separation distance. However there is a sudden jump of the viscous force before it gradually increases, then it decreases dramatically. The influences of various parameters on the separation time were investigated. It was found that the separation time increases with the mass of the upper plate, surface tension, viscosity and volume of the liquid. The separation time decreases with the initial distance and the contact angles of the liquid.

Degradation of Hydrophobic Surface Coatings Under Water Exposure

2018 ◽  
Author(s):  
Matthew A. Trapuzzano ◽  
Rasim Guldiken ◽  
Andrés Tejada-Martínez ◽  
Nathan B. Crane
Keyword(s):  
Contact Angle ◽  
Material Selection ◽  
Contact Angle Hysteresis ◽  
Hydrophobic Surface ◽  
Contact Angles ◽  
Water Exposure ◽  
Angle Measurements ◽  
Degradation Rates ◽  
Contact Angle Measurements ◽  
Receding Contact

Many important processes depend on the wetting of liquids on surfaces. Wetting is commonly controlled through material selection, coatings, and/or surface texture, however these means are sensitive to environmental conditions. Some “hydrophobic” fluoropolymer coatings are sensitive to extended water exposure as evidenced by declining contact angles and increasing contact angle hysteresis. Understanding degradation of these coatings is critical to processes that employ them. To accomplish this, contact angle measurements were taken before, during, and after slides coated with FluoroSyl 3750 or Cytop were submerged in water, or vibrated while covered in water. Both methods demonstrated similar changes in advancing contact angle though vibration increased degradation rates significantly. However, it does not simply accelerate the process as different trends are apparent in receding contact angles. The FluoroSyl 3750 showed no clear degradation under either condition. Surface profilometry did not detect any surface morphology differences that might cause contact angle change.

Wetting and Capillarity Effects On Bubble Formation From Orifice Plates Submerged in Pools of Water

2021 ◽  
Author(s):  
Sanjivan Manoharan ◽  
Raj M. Manglik ◽  
Milind A. Jog
Keyword(s):  
Contact Angle ◽  
Bubble Growth ◽  
High Speed ◽  
Bubble Formation ◽  
Minimum Energy ◽  
Hydrophobic Surface ◽  
Contact Angles ◽  
Orifice Diameter ◽  
Capillary Length ◽  
Critical Contact

Abstract An experimental study of bubble growth from submerged orifice plates in pools of water is carried out to scale and correlate the effects of surface wettability and orifice diameter D0 on ebullience. Measurements of bubble growth on surfaces with nine different contact angles (38° ≤ θ ≤ 128°) with varying air flow rates (1 to 300 ml/min) were made using high speed videography and image processing. In the static or constant-volume regime, below a critical contact angle θc, the bubble base remains attached to the orifice and the equivalent departure diameter Db is independent of contact angle θ. On the other hand, above the critical contact angle, the bubble base spreads on the surface resulting in larger Db. For θ > θc, Db is strongly dependent on θ and increases with it. Using minimum energy method, it is shown that the wettability effects can be scaled and correlated by a modified capillary length, defined as a function of the Laplace length and contact angle. The proposed correlation provides predictions of Db that agree with experimental data of this study as well as those available in the literature to within ±15 %. Moreover, for a hydrophobic surface when D0 > twice the modified capillary length, the bubble grows inside the orifice; for a hydrophilic surface this scales with twice the capillary length and effect of θ is not seen.

Roughness Effects on Continuous and Discrete Flows in Superhydrophobic Microchannels

2011 ◽  
Vol 9 (5) ◽  
pp. 1094-1105 ◽  
Author(s):  
Junfeng Zhang ◽  
Daniel Y. Kwok
Keyword(s):  
Surface Roughness ◽  
Contact Angle ◽  
Slip Length ◽  
Contact Angle Hysteresis ◽  
Hydrophobic Surface ◽  
Contact Angles ◽  
Lattice Boltzmann Model ◽  
Roughness Effects ◽  
Apparent Slip ◽  
Velocity Changes

AbstractThe dynamic behaviors of continuous and discrete flows in superhydrophobic microchannels are investigated with a lattice Boltzmann model. Typical characters of the superhydrophobic phenomenon are well observed from our simulations, including air trapped in the surface microstructures, high contact angles, low contact angle hysteresis, and reduced friction to fluid motions. Increasing the roughness of a hydrophobic surface can produce a large flow rate through the channel due to the trapped air, implying less friction or large apparent slip. The apparent slip length appears to be independent to the channel width and could be considered as a surface property. For a moving droplet, its behavior is affected by the surface roughness from two aspects: the contact angle difference between its two ends and the surface-liquid interfacial friction. As a consequence, the resulting droplet velocity changes with the surface roughness as firstly decreasing and then increasing. Simulation results are also compared with experimental observations and better agreement has been obtained than that from other numerical method. The information from this study could be valuable for microfluidic systems.

scholarly journals Effect of surface roughness on the angular acceleration for a droplet on a super-hydrophobic surface

Friction ◽  
2020 ◽  
Author(s):  
Longyang Li ◽  
Jingfang Zhu ◽  
Zhixiang Zeng ◽  
Eryong Liu ◽  
Qunji Xue
Keyword(s):  
Contact Angle ◽  
High Speed ◽  
Adhesion Force ◽  
Angular Acceleration ◽  
Hydrophobic Surface ◽  
Adhesive Force ◽  
Industrial Applications ◽  
Contact Angles ◽  
Etching Time ◽  
Receding Contact

Abstract The motion of droplets on a super-hydrophobic surface, whether by sliding or rolling, is a hot research topic. It affects the performance of super-hydrophobic materials in many industrial applications. In this study, a super-hydrophobic surface with a varied roughness is prepared by chemical-etching. The adhesive force of the advancing and receding contact angles for a droplet on a super-hydrophobic surface is characterized. The adhesive force increases with a decreased contact angle, and the minimum value is 0.0169 mN when the contact angle is 151.47°. At the same time, the motion of a droplet on the super-hydrophobic surface is investigated by using a high-speed camera and fluid software. The results show that the droplet rolls instead of sliding and the angular acceleration increases with an increased contact angle. The maximum value of the angular acceleration is 1,203.19 rad/s2 and this occurs when the contact angle is 151.47°. The relationship between the etching time, roughness, angular acceleration, and the adhesion force of the forward and backward contact angle are discussed.

Comparing Electrowettability and Surfactants As Tools for Wettability Enhancement on a Hydrophobic Surface

2019 ◽  
Author(s):  
Manojkumar Lokanathan ◽  
Himanshu Sharma ◽  
Mostafa Shabaka ◽  
Vaibhav Bahadur ◽  
Kishore Mohanty
Keyword(s):  
Contact Angle ◽  
Electric Fields ◽  
Energy Production ◽  
Electrical Potential ◽  
Hydrophobic Surface ◽  
Contact Angles ◽  
Wettability Alteration ◽  
Electrical Potential Difference ◽  
Water Droplets ◽  
Zwitterionic Surfactants

Abstract Wettability alteration has significant applications in microfluidics, energy production and process engineering. Surfactants have been widely used for wettability alteration on surfaces. More recently, electrowetting (EW) has emerged as a powerful microfluidic technique to dynamically alter wettability. EW relies on the application of an electrical potential difference across a dielectric layer on which the fluid rests. This work analyzes the extent of wettability enhancement of water droplets on a hydrophobic surface (in air) via the use of surfactants and EW. Nine surfactants were chosen from the categories of anionic, cationic and zwitterionic surfactants. The critical micelle concentration (CMC) of these surfactants, and the wettability of surfactant-infused water droplets was measured at post and pre-CMC concentrations. Next, experiments were conducted to quantify the wettability enhancement of water droplets (with surfactants) via EW. Many interesting insights on the interplay between surfactants and electric fields are uncovered in this work. As expected, adding surfactants enhances wettability up to the CMC. EW can further enhance wettability of surfactant solutions and further reduce the contact angle by as much as 30°. Interestingly, it is seen that the influence of EW in enabling CA reduction is reduced by the addition of surfactants at pre-CMC levels. Conversely, surfactants strengthen the influence of EW at higher concentrations. It is noted that the extent of wettability alteration via EW is limited by the phenomena of contact angle saturation, wherein the contact angle saturates beyond a certain voltage. Interestingly, it is seen that at post CMC concentrations, the saturation contact angles are independent of surfactant concentrations.

Temperature Dependence of the Contact Angle of Water on a Hydrophobic Surface at Pressurized Condition

2020 ◽  
Author(s):  
Jia-Wen Song ◽  
Meng-Chen Ma ◽  
Li-Wu Fan
Keyword(s):  
Surface Tension ◽  
Contact Angle ◽  
Temperature Dependence ◽  
Sessile Drop ◽  
Hydrophobic Surface ◽  
Contact Angles ◽  
Surface Wettability ◽  
Surface Thermodynamics ◽  
Temperature Spectrum ◽  
Phase Change Heat Transfer

Abstract It is of both practical and scientific significance to study the temperature dependence of contact angles, towards development of surface wettability manipulation techniques for enhanced phase change heat transfer as well as the theoretical estimation of solid interfacial energy. However, the variations of surface wettability of a hydrophobic solid with altering temperature remain unclear. In this work, in situ characterizations of the contact angle of water on Teflon (PTFE) surfaces as well as the surface tension of water over a temperature spectrum from ∼25 °C to 160 °C at pressurized condition (2 MPa) were conducted by employing the sessile drop and pendant drop methods, respectively. A nearly invariant trend of the contact angle of water was observed over the entire temperature range. Moreover, it was shown that the surface tension of water linearly declines with raising the temperature. Based on the theory of surface thermodynamics, the effects of temperature on the contact angles were analyzed with the variations of interfacial tensions.

Fabrication on Hydrophobicity of the Etched Aluminium Alloy Surfaces

2014 ◽  
Vol 924 ◽  
pp. 134-137
Author(s):  
Jing Li ◽  
Guo Hua Cao ◽  
Xin Ming Zhang ◽  
Cheng Yu Xu ◽  
Qiang Li
Keyword(s):  
Contact Angle ◽  
Aluminum Alloy ◽  
Water Contact Angle ◽  
Laser Scanning ◽  
Hydrophobic Surface ◽  
Laser Scanning Confocal Microscopy ◽  
Contact Angles ◽  
Angle Measurement ◽  
Organic Liquids ◽  
Water Contact

Hydrophobic surfaces with contact angles greater than 90° and roll-off angles below 10° for water have been developed, based on low energy surfaces and rough texture on the micro-and nanometer scales. In this study, we fabricated a hydrophobic surface on a aluminum alloy substrate using the method of chemical etching without being modified by organic liquids such as surfactant-based solutions, alcohols, or alkanes. The measurement showed that the as-prepared surfaces possessed roughness on the micrometer scales by laser scanning confocal microscopy. The etched aluminum alloy surfaces had a maximum water contact angle of 120o by using a water contact angle measurement. The forming course of the aluminum alloy etched surfaces with pores was analyzed. The wettability of the etched aluminum alloy surfaces is reinforced by means of controlling the surface rough texture on the micrometer scales.

scholarly journals Influence of moisture content on the contact angle and surface tension measured on birch wood surfaces

2021 ◽  
Author(s):  
Rami Benkreif ◽  
Fatima Zohra Brahmia ◽  
Csilla Csiha
Keyword(s):  
Surface Tension ◽  
Contact Angle ◽  
Moisture Content ◽  
Solid Wood ◽  
Contact Angles ◽  
Surface Moisture ◽  
Wood Coatings ◽  
The Relationship ◽  
Wood Surfaces

AbstractSurface tension of solid wood surfaces affects the wettability and thus the adhesion of various adhesives and wood coatings. By measuring the contact angle of the wood, the surface tension can be calculated based on the Young-Dupré equation. Several publications have reported on contact angle measured with different test liquids, under different conditions. Results can only be compared if the test conditions are similar. While the roles of the drop volume, image shooting time etc., are widely recognized, the role of the wood surface moisture content (MC) is not evaluated in detail. In this study, the effect of wood moisture content on contact angle values, measured with distilled water and diiodomethane, on sanded birch (Betula pendula) surfaces was investigated, in order to find the relationship between them. With increasing MC from approximately 6% to 30%, increasing contact angle (decreasing surface tension) values were measured according to a logarithmic function. The function makes possible the calculation of contact angles that correspond to different MCs.

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