H. Yildirim Erbil

Evaporation of pure liquid sessile and spherical suspended drops: A review

A sessile drop is an isolated drop which has been deposited on a solid substrate where the wetted area is limited by a contact line and characterized by contact angle, contact radius and drop height. Diffusion-controlled evaporation of a sessile drop in an ambient gas is an important topic of interest because it plays a crucial role in many scientific applications such as controlling the deposition of particles on solid surfaces, in ink-jet printing, spraying of pesticides, micro/nano material fabrication, thin film coatings, biochemical assays, drop wise cooling, deposition of DNA/RNA micro-arrays, and manufacture of novel optical and electronic materials in the last decades. This paper presents a review of the published articles for a period of approximately 120 years related to the evaporation of both sessile drops and nearly spherical droplets suspended from thin fibers. After presenting a brief history of the subject, we discuss the basic theory comprising evaporation of micrometer and millimeter sized spherical drops, self cooling on the drop surface and evaporation rate of sessile drops on solids. The effects of drop cooling, resultant lateral evaporative flux and Marangoni flows on evaporation rate are also discussed. This review also has some special topics such as drop evaporation on superhydrophobic surfaces, determination of the receding contact angle from drop evaporation, substrate thermal conductivity effect on drop evaporation and the rate evaporation of water in liquid marbles.

Evaporation Rate of Graphite Liquid Marbles: Comparison with Water Droplets

Liquid marbles are liquid drops made completely nonwetting by encapsulating the drop with a hydrophobic powder. The absence of contact with the substrate avoids contamination problems and produces high marble displacement velocities. Liquid marbles behave as microreservoirs of liquids able to move without any leakage and are promising candidates to be applied in biomedical and genetic analysis where 2D microfluidics and lab-on-a-chip methods are used. The lifetime of a liquid marble depends on the chemical nature and particle size of the hydrophobic powder as well as the liquid used to form it. There is a need for chemically inert liquid marbles, which can be used over sufficiently long periods for industrial applications. In this work, we successfully synthesized graphite liquid marbles for the first time by encapsulating graphite micropowder on water droplets and determined their evaporation periods and useful lifetimes in constant relative humidity and temperature conditions in a closed chamber. The evaporation rates of graphite liquid marbles were compared with the rates of pure water droplets in the same conditions, and it was found that they had nearly twice the lifetime of pure water droplets. The use of chemically inert graphite particles having electrical conductivity and dry lubrication properties to form a liquid marble may be a starting point for their successful use in microfluidics, genetic analysis, antifouling, wear-free micromachine, electromechanical actuator, and valve applications.

Range of applicability of the Wenzel and Cassie-Baxter equations for superhydrophobic surfaces.

The Wenzel and Cassie-Baxter equations depending on the extent of liquid/solid interfacial contact area were generally used to estimate water contact angles on superhydrophobic surfaces. In this study, a simple method is proposed on the criterion to use the Wenzel and Cassie-Baxter equations to evaluate the contact angle results on superhydrophobic surfaces. In this method, the difference between the theoretical (geometric) and experimental contact angle-dependent Wenzel roughness parameter, Delta r(w), and Cassie-Baxter solid/liquid contact area fraction, Delta f(s)(CB) were determined, and the validity of these equations was evaluated. We used the data of eight recent publications where the water drop sits on square and cylindrical pillar structured superhydrophobic model surfaces. We evaluated the contact angle results of 166 patterned samples with our method. We also found that the effect of contact angle error margins was low to vary these parameters. In general, the use of the Wenzel equation was found to be wrong for most of the samples (74% of the samples for cylindrical and 58% for square pillar patterned surfaces), and the deviations from the theory were also high for the remaining (26% for cylindrical and 42% for square) samples, and it is concluded that the Wenzel equation cannot be used for superhydrophobic surfaces other than a few exceptions, especially for cylindrical patterns. For the Cassie-Baxter equation, two situations are possible: for positive Delta f(s)(CB), there is only a partial contact of the drop with the top solid surface, and, for negative Delta f(s)(CB), the penetration of the drop in between the pillars is possible, and thus the liquid drop is in contact with the lateral sides of the pillars. We found that 65% of the samples containing cylindrical pillars (52-77% with error margins) and 44% of the samples containing square pillars (38-50% with error margins) resulted in negative Delta f(s)(CB)(red) values. In addition, large deviations of experimental water contact angle results, theta r(e) from the theoretical theta r(CB) were also determined for most of the samples, indicating that the Cassie-Baxter equation should be applied to superhydrophobic surfaces with caution.

Surface energetics of films of poly(vinyl acetate—butyl acrylate) emulsion copolymers

Abstract Vinyl acetate (VA) and butyl acrylate (BA) monomers were emulsion copolymerized by using controlled monomer and initiator addition method. Particle size distributions, viscosity average molecular weights ( M V ), glass-transition temperatures (Tg°) and molar composition of the copolymers were determined. Surface tension components of the copolymer films were calculated from contact angle data of various liquids by using Van Oss—Good methodology. A decrease of average particle size and Tg° was found by the increase of BA content. M V increased up to 40mol% BA, then decreased. Apolar Lifshitz—Van der Waals surface tension component was not affected by the change in BA content; however, electron donor surface tension coefficient (γs−) decreased considerably with the BA increase. The work of adhesion was calculated for cellulosic substrate—latex film interfaces.

The lifetime of floating liquid marbles: the influence of particle size and effective surface tension

In this study, we investigated the factors affecting the lifetime of liquid marbles placed on a glass surface and floating on water. It was found that the particle size, surface free energy and hydrophobicity of the encapsulating microparticles determine the effective surface tension and lifetime of a liquid marble floating on water. We formed liquid marbles using ultra-hydrophobic poly(perfluoroalkyl ethyl acrylate) powder with three different particle sizes (8 ± 1, 20 ± 2 and 60 ± 5 μm) and polytetrafluoroethylene powder (7 ± 2 μm). It was found that both the effective surface tension and lifetime of a floating liquid marble increased considerably with the decrease in the particle size of the ultra-hydrophobic poly(perfluoroalkyl ethyl acrylate) powder. We also determined that a floating liquid marble had a longer lifetime if the water contact angle of the polymer powder was high and its surface free energy was low by comparing the results of two different powders having very close average particle sizes but different hydrophobicities.

Diffusion-controlled evaporation of sodium dodecyl sulfate solution drops placed on a hydrophobic substrate

In this work, the effect of SDS anionic surfactant on the diffusion-controlled evaporation rate of aqueous solution drops placed on TEFLON-FEP substrate was investigated with 11 different SDS concentrations. Drop evaporation was monitored in a closed chamber having a constant RH of 54-57% by a video camera. The initial contact angle, θ(i) decreased from 104±2° down to 68±1° due to the adsorption of SDS both at the water-air and the solid-water interfaces. The adsorption of SDS on the solid surface was found to be 76% of that of its adsorption at the water-air interface by applying Lucassen-Reynders approach. An equation was developed for the comparison of the evaporation rates of drops having different θ(i) on the same substrate. It was found that the addition of SDS did not alter the drop evaporation rate considerably for the first 1200 s for all the SDS concentrations. The main difference was found to be the change of the mode of drop evaporation by varying the SDS concentration. The constant θ mode was operative up to 80 mM SDS concentration, whereas constant contact area mode was operative after 200 mM SDS concentrations due to rapid drop pining on the substrate.

Improved Icephobic Properties on Surfaces with a Hydrophilic Lubricating Liquid

Slippery liquid-infused porous surfaces were developed recently for icephobic surface applications. Perfluorinated liquids, silicone oil, hydrocarbon, and water were used as lubricating liquids to form a continuous layer on a suitable substrate to prevent icing. However, ice accretion performances of these surfaces have not been reported previously depending on the type of the lubricant. In this work, fluorinated aliphatics, polyalphaolefin, silicone oil, and decamethylcyclopenta siloxane were used as hydrophobic lubricants; water, ethylene glycol, formamide, and water-glycerine mixture were used as hydrophilic lubricants to be impregnated by hydrophobic polypropylene and hydrophilic cellulose-based filter paper surfaces; ice accretion, drop freezing delay time, and ice adhesion strength properties of these surfaces were examined; and the results were compared to those of the reference surfaces such as aluminum, copper, polypropylene, and polytetrafluoroethylene. An ice accretion test method was also developed to investigate the increase of the mass of formed ice gravimetrically by spraying supercooled water onto these surfaces at different subzero temperatures ranging between -1 and -5 °C. It was determined that hydrophilic solvents (especially a water-glycerine mixture) that impregnated hydrophilic porous surfaces would be a promising candidate for anti-icing applications at -2 °C and 56-83% relative humidity because ice accretion and ice adhesion strength properties of these surface decreased simultaneously in these conditions.

Effect of contact angle hysteresis on the removal of the sporelings of the green alga Ulva from the fouling-release coatings synthesized from polyolefin polymers

Wettability is one of the surface characteristics that is controlled by the chemical composition and roughness of a surface. A number of investigations have explored the relationship between water contact angle and surface free energy of polymeric coatings with the settlement (attachment) and adhesion strength of various marine organisms. However, the relationship between the contact angle hysteresis and fouling-release property is generally overlooked. In the present work, coatings were prepared by using commercial hydrophobic homopolymer and copolymer polyolefins, which have nearly the same surface free energy. The effects of contact angle hysteresis, wetting hysteresis, and surface free energy on the fouling-release properties for sporelings of the green alga Ulva from substrates were then examined quantitatively under a defined shear stress in a water channel. The ease of removal of sporelings under shear stress from the polymer surfaces was in the order of PP>HDPE>PPPE>EVA-12 and strongly and positively correlated with contact angle and wetting hysteresis; i.e., the higher the hysteresis, the greater the removal.

Determination of surface free energy components of polymers from contact angle data using nonlinear programming methods

Abstract A nonlinear method was applied to determine the surface free energy components γpSV and γdSV of ethylene—vinyl acetate and ethylene—vinyl alcohol copolymers from contact angle data by using the geometric-mean approximation. With this method, the “actual” surface free energy component of the probe liquids, ḡgpLV and ḡgdLV, which reflect the fullest extent of liquid—polymer interactions and the interaction parameters, Φ, were also simultaneously determined. The critical surface free energy γc of the copolymers was calculated from Φav values by using Goods equation.

Surface free energy analysis of polystyrene–poly(β-hydroxynonanoate) graft copolymers

Polystyrene–poly(β-hydroxynonanoate) (PS–PHN) graft copolymers having 12, 17, 27, 31, 37, 60, 89, and 94% PS contents were prepared using active PS synthesized from styrene and oligododecandioyl peroxide initiator. Then the copolymer films were cast from chlo-roform, toluene, cyclohexanone, and butyl acetate. PS and PHN homopolymer blends were also prepared as films having 8, 12, 28, 34, 44, and 64% PS in the same casting solvents. The contact angles of water, glycerol, ethylene glycol, formamide, trans-decaline, and paraffin oil drops were measured on these cast films. The contact angle results were evaluated in terms of surface free energy components using Van Oss–Good methodology. It was determined that the Lifshitz–van der Waals component (γ) of the graft copolymers and blend of homopolymers did not differ so much with the copolymer composition. However, the electron donor component of the surface free energy (γ) decreased with the decrease of PHN content to a considerable extent. It was observed that this decrease is more linear than that of homopolymer blends. The effect of the nature of the casting solvents on the surface properties of the polymers was also investigated in terms of their solubility parameter values and it was found that the more H-bonding solvents resulted in higher (γ−s) surfaces.