Water–propylene glycol sessile droplet shapes and migration: Marangoni mixing and separation of scales

2022 ◽  
Vol 933 ◽  
Author(s):  
J. Charlier ◽  
A.Y. Rednikov ◽  
S. Dehaeck ◽  
P. Colinet ◽  
D. Terwagne
Keyword(s):  
Contact Angle ◽  
Propylene Glycol ◽  
Ambient Air ◽  
Contact Angles ◽  
Apparent Contact Angle ◽  
Sessile Droplet ◽  
Convective Mixing ◽  
Separation Of Scales ◽  
And Migration

New light is shed on morphological features of water–propylene glycol sessile droplets evaporating into ambient air at not too high relative humidity. Such droplets adopt a Marangoni-contracted shape even on perfectly wetting substrates, an effect well known since Cira et al. (Nature, 519, 2015). We here highlight a strong separation of scales normally occurring for such droplets. Namely, there is a narrow high-curvature zone localized at the foot of the droplet, where the apparent contact angle is formed, while the core of the droplet merely adheres to the classical (capillary–gravity) static shape. Experimentally, we rely upon interferometry to discern such fine key details. We detect a maximum of the droplet slope profile in the foot region, which amounts to the apparent contact angle. Theoretically, a local description of the foot region is devised. We indicate a crucial role of convective mixing by the solutal Marangoni flow, here accounted for by the Taylor dispersion, which proves to underlie the separation of scales and ensure self-consistency of the local model. Migration of such droplets in a humidity gradient is also approached within the same experimental and theoretical framework. It is considered that the resulting back–front asymmetry of the apparent contact angles drives the motion similarly to a wettability gradient, although the drag (‘Cox–Voinov’) factor is here found to be different. The predictions, comparing well with the measurements (our own and from the literature), are based on rigorous models, isothermal and as reduced as possible, without any fitting parameters or microphysics effects.

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.

Ring-Shaped Sessile Droplet

2011 ◽  
Author(s):  
Svyatoslav S. Chugunov ◽  
Douglas L. Schulz ◽  
Iskander S. Akhatov
Keyword(s):  
Contact Angle ◽  
Liquid Droplet ◽  
Liquid Structure ◽  
Solid Substrate ◽  
Contact Angles ◽  
Equilibrium Contact Angle ◽  
Sessile Droplet ◽  
System Equilibrium ◽  
Energy Consideration ◽  
External Forces

It is recognized that small liquid droplet placed on the solid substrate forms equilibrium contact angle that can be obtained from well-known Young’s law. Previously, deviations from Young’s law were demonstrated for the droplets exposed to external fields (gravity, electric, etc) and for the droplets on non-homogeneous substrates. This work reveals that the Young’s equilibrium contact angle can be altered by geometrical reasons only. We consider a ring-shaped droplet on a solid substrate as a test structure for our discussion. We use the global energy consideration for analysis of system equilibrium for the case of freely deposited liquid with no external forces applied. The theoretical analysis shows that steady ring-shaped liquid structure on a solid substrate does exist with contact angles on both contact lines to be different from the Young’s equilibrium contact angle.

scholarly journals The Role of Contact Angle and Pore Width on Pore Condensation and Freezing

2019 ◽  
Author(s):  
Robert O. David ◽  
Jonas Fahrni ◽  
Claudia Marcolli ◽  
Fabian Mahrt ◽  
Dominik Brühwiler ◽  
...  
Keyword(s):  
Contact Angle ◽  
Active Sites ◽  
Water Saturation ◽  
Capillary Condensation ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Pore Width ◽  
Kelvin Effect ◽  
Pore Condensation

Abstract. It has recently been shown that pore condensation and freezing (PCF) is a mechanism responsible for ice formation under cirrus cloud conditions. PCF is defined as the condensation of liquid water in narrow capillaries below water saturation due to the Kelvin effect, followed by either heterogeneous or homogeneous nucleation depending on the temperature regime and presence of an ice nucleating active site. By using sol-gel synthesized silica with well-defined pore diameters, morphology and distinct chemical surface-functionalization, the role of the water-silica contact angle and pore width on PCF is investigated. We find that contact angle and pore width play an important role in determining the relative humidity required for capillary condensation as predicted by the Kelvin effect and subsequent ice nucleation at cirrus temperatures. For the pore diameters and contact angles covered in this study, 2.2–9.2 nm and 15–78°, respectively, our results reveal that the contact angle plays an important role in predicting the humidity required for pore filling while the pore diameter determines the ability of pore water to freeze. For T > 235 K and below water saturation, pore diameters and contact angles were not able to predict the freezing ability of the particles suggesting an absence of active sites, thus ice nucleation did not proceed via a PCF mechanism. Rather, the ice nucleating ability of the particles depended solely on chemical functionalization. Therefore, parameterizations for the ice nucleating abilities of particles at cirrus conditions should differ from parameterizations at mixed-phase clouds conditions. Our results support PCF as the atmospherically relevant ice nucleation mechanism below water saturation when porous surfaces are encountered in the troposphere.

Microstructure and Wettability on the Elytral Surface of Aquatic Beetle

2013 ◽  
Vol 461 ◽  
pp. 731-740 ◽  
Author(s):  
Ming Xia Sun ◽  
Ai Ping Liang ◽  
Gregory S. Watson ◽  
Jolanta A. Watson ◽  
Yong Mei Zheng ◽  
...  
Keyword(s):  
Contact Angle ◽  
Contact Angles ◽  
Vital Role ◽  
Butterfly Wing ◽  
Scanning Microscope ◽  
Optical Contact ◽  
Three Phase ◽  
Aquatic Beetle ◽  
Wing Scales

The microstructures on elytral surface of aquatic beetles belonging to Hydrophilidae and Dytiscidae were observed under an environment scanning microscope, and the wettabilities were determined with an optical contact angle meter. The results show the elytral surfaces are relatively smooth compared to the structures of other insects such as the butterfly wing scales or cicada wing protrusions. They exhibit a polygonal structuring with grooves and pores being the main constituent units. The contact angles (CAs) range from 47.1oto 82.1o. The advancing and receding angles were measured by injecting into and withdrawing a small amount of water on the most hydrophilic (with a contact angle of 47.1o) and hydrophobic (with a contact angle of 82.1o) elytral surfaces, which illustrates the vital role of three-phase contact line (TCL) in the wetting mechanism of aquatic beetle elytral surfaces.

scholarly journals On the contact region of a diffusion-limited evaporating drop: a local analysis

2013 ◽  
Vol 739 ◽  
pp. 308-337 ◽  
Author(s):  
S. J. S. Morris
Keyword(s):  
Contact Angle ◽  
Boundary Value Problem ◽  
Sessile Drop ◽  
Contact Region ◽  
Boundary Value ◽  
Dynamic Contact Angle ◽  
Dynamic Contact ◽  
Apparent Contact Angle ◽  
Local Analysis ◽  
Separation Of Scales

AbstractMotivated by experiments showing that a sessile drop of volatile perfectly wetting liquid initially advances over the substrate, but then reverses, we formulate the problem describing the contact region at reversal. Assuming a separation of scales, so that the radial extent of this region is small compared with the instantaneous radius$a$of the apparent contact line, we show that the time scale characterizing the contact region is small compared with that on which the bulk drop is evolving. As a result, the contact region is governed by a boundary-value problem, rather than an initial-value problem: the contact region has no memory, and all its properties are determined by conditions at the instant of reversal. We conclude that the apparent contact angle$\theta $is a function of the instantaneous drop radius$a$, as found in the experiments. We then non-dimensionalize the boundary-value problem, and find that its solution depends on one parameter$\mathscr{L}$, a dimensionless surface tension. According to this formulation, the apparent contact angle is well-defined: at the outer edge of the contact region, the film slope approaches a limit that is independent of the curvature of bulk drop. In this, it differs from the dynamic contact angle observed during spreading of non-volatile drops. Next, we analyse the boundary-value problem assuming$\mathscr{L}$to be small. Though, for arbitrary$\mathscr{L}$, determining$\theta $requires solving the steady diffusion equation for the vapour, there is, for small$\mathscr{L}$, a further separation of scales within the contact region. As a result,$\theta $is now determined by solving an ordinary differential equation. We predict that$\theta $varies as${a}^{- 1/ 6} $, as found experimentally for small drops ($a\lt 1~\mathrm{mm} $). For these drops, predicted and measured angles agree to within 10–30 %. Because the discrepancy increases with$a$, but$\mathscr{L}$is a decreasing function of$a$, we infer that some process occurring outside the contact region is required to explain the observed behaviour of larger drops having$a\gt 1~\mathrm{mm} $.

Metrics for Quantifying Surface Wetting Effects on Vaporization Processes at Nanostructured Hydrophilic Surfaces

2016 ◽  
Author(s):  
Claire M. Kunkle ◽  
Van P. Carey
Keyword(s):  
Contact Angle ◽  
Contact Line ◽  
Contact Angles ◽  
Apparent Contact Angle ◽  
Static Contact Angle ◽  
Sensitive Indicator ◽  
Nanostructured Surfaces ◽  
Hydrophilic Surfaces ◽  
Static Contact ◽  
Spread Area

A static contact angle is most often used as a means of quantifying the wetting characteristics of the liquid phase in vaporization processes at a solid surface. This metric is often convenient to measure and intuitive in its interpretation, but when a surface is superhydrophilic, the resulting low contact angles are difficult to measure accurately from photographs of sessile droplet profiles or contact line regions. For droplets at ultra low contact angles, small changes of contact angle can produce very large changes in wetted surface area, which makes small uncertainties in contact angle result in large uncertainties in wetted area. For hydrophilic nanostructured surfaces, another disadvantage is that the relationship of the macroscopic (apparent) contact angle to the nanoscale interaction of the liquid and vapor contact line with the nanostructured surface is not always clear. In this study, a new wetting metric based on spreading characteristics of sessile droplets is proposed that can be easily measured for hydrophilic surfaces. This metric also has the advantage that it is a more direct and sensitive indicator of how a droplet spreads on the surface. The spread area directly impacts heat transfer interactions between the droplet and the surface, therefore affecting evaporation time. Consequently, a metric that more directly illustrates the spread area provides an indication of how the wetting will affect these mechanisms. Use of the proposed new metric is explored in the context of evaporation and boiling applications with superhydrophilic surfaces. Characteristics of this metric are also compared to static contact angle and other choices of wetting metrics suggested in earlier studies, such as dynamic advancing and receding contact angles, and spreading coefficients. The effects of nanoscale structure and/or roughness on the proposed wetting metric are analyzed in detail. A model is developed that predicts the dependence of the proposed wetting parameter on intrinsic material wettability for rough, nano-structured surfaces. The model results demonstrate that the proposed metric is a more sensitive indicator of macroscopic wetting behavior than apparent contact angle when the surface is superhydrophilic. This characteristic of the proposed new metric is shown to have advantages over other wetting metrics in the specific case of superhydrophilic nanostructured surfaces. Application of the proposed wetting metric is demonstrated for some example nanostructured surfaces. The results of our study indicate that this proposed new metric can be particularly useful for characterizing the effects of variable wetting on vaporization processes on highly wetted nanostructured surfaces.

scholarly journals Role of Intermolecular Forces on the Contact Angle of Vegetable Oil Droplets during the Cooling Process

2018 ◽  
Vol 2018 ◽  
pp. 1-8 ◽  
Author(s):  
Muhammad Akhlis Rizza ◽  
Widya Wijayanti ◽  
Nurkholis Hamidi ◽  
I. N. G. Wardana
Keyword(s):  
Surface Tension ◽  
Contact Angle ◽  
Hydrogen Bonds ◽  
Vegetable Oil ◽  
Intermolecular Forces ◽  
Contact Angles ◽  
Droplet Surface ◽  
Water Molecules ◽  
Oil Droplets

This study aims to experimentally determine the role of intermolecular forces on the contact angle of vegetable oil droplets. Contact angles were recorded using a microscope and measured using digital software. The results show that the surface tension of vegetable oils is influenced by the London force between the electron clouds of molecules. The process of cooling increases vegetable oil contact angles, due to the decreased kinetic energy of constituent molecules and increased London force on the molecules. A decrease in temperature causes the surrounding water vapor to condense, which adheres to the droplet surface (due to the hydrophilic properties of molecules). Hydrogen bonds develop after moisture adheres to the surface. Further, water molecules on the droplet surface reduce the surface tension, because of hydrogen bonds between the molecules on the droplet surface and moisture. Hydrogen bonds among the molecules force water molecules to accumulate on the droplet surface, which suppresses the droplet surface; therefore the contact angle decreases.

Investigation of Pre-Tungsten Silicide Deposition Wet Chemical Processing

1995 ◽  
Vol 386 ◽  
Author(s):  
A. Philipossian ◽  
M. Moinpour ◽  
R. Wilkinson ◽  
V. H. C. Watt
Keyword(s):  
Contact Angle ◽  
Polycrystalline Silicon ◽  
Silicon Surface ◽  
Morphological Characteristics ◽  
Ambient Air ◽  
Contact Angles ◽  
Silicon Surfaces ◽  
Native Oxide ◽  
Initial Contact ◽  
Air Exposure

ABSTRACTRemoving the native oxide from the poly-Si surface prior to WSix deposition is essential for achieving high quality silicides as well as sufficient film adhesion, particularly after high temperature anneal or oxidation. Contact angle studies have been used to determine initial and time-dependent surface characteristics of several types of silicon surfaces following immersions in HF-based etchants for varying amounts of time. The morphological characteristics of the surfaces before and after exposure to etchants, as well as the relative etch rates and wetting capabilities of the etchants have been used to explain the following results: With respect to initial contact angle studies, the implanted & annealed polycrystalline silicon surface has the lowest contact angle followed by polycrystalline and monocrystalline surfaces. Longer immersion times yield lower initial contact angles. The 0.1% lightly-buffered HF solution results in the highest contact angle followed by the 1% buffered HF solution with surfactant, and the 1% HF solution. With respect to contact angle changes during ambient air exposure time, the asdeposited polycrystalline silicon surface is most stable followed by monocrystalline, and implanted & annealed polycrystalline silicon surfaces. Longer immersion times improve surface stability while the 0.1% lightly-buffered HF solution results in the most stable surface followed by the 1% buffered HF solution with surfactant, and the 1% HF solution.

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.

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