Water Drop Evaporation on Slippery Liquid-Infused Porous Surfaces (SLIPS): Effect of Lubricant Thickness, Viscosity, Ridge Height, and Pattern Geometry.

Sessile drop evaporation and condensation on slippery liquid-infused porous surfaces (SLIPS) is crucial for many applications. However, its modeling is complex since the infused lubricant forms a wetting ridge around the drop close to the contact line, which partially blocks the free surface area and decreases the drop evaporation rate. Although a good model was available after 2015, the effects of initial lubricant heights (hoil)i above the pattern, and the corresponding initial ridge heights (hr)i, lubricant viscosity, and solid pattern type were not well studied. This work fills this gap where water drop evaporations from SLIPS, which are obtained by infusing silicone oils (20 and 350 cSt) onto hydrophobized Si wafer micropatterns having both cylindrical and square prism pillars, are investigated under constant relative humidity and temperature conditions. With the increase of (hoil)i, the corresponding (hr)i increased almost linearly on lower parts of the drops for all SLIPS samples, resulting in slower drop evaporation rates. A novel diffusion-limited evaporation equation from SLIPS is derived depending on the available free liquid-air interfacial area, ALV, which represents the unblocked part of the total drop surface. The calculation of the diffusion constant, D, of water vapor in air from (dALV/dt) values obtained by drop evaporation was successful up to a threshold value of (hoil)i = 8 µm within ±7%, and large deviations (13-27%) were obtained when (hoil)i > 8 µm, possibly due to the formation of thin silicone oil cloaking layers on drop surfaces, which partially blocked evaporation. The increase of infused silicone oil viscosity caused only a slight increase (12-17%) in drop lifetimes. The effects of the geometry and size of the pillars on the drop evaporation rates were minimal. These findings may help optimize the lubricant oil layer thickness and viscosity used for SLIPS to achieve low operational costs in the future.

Wetting of Superhydrophobic Polylactic Acid Micropillared Patterns.

Superhydrophobic (SH) polylactic acid (PLA) surfaces were previously produced by various methods and used especially in biomedical applications and oil/water separation processes after 2008. However, the wettability of SH-PLA patterns containing micropillars has not been investigated before. In this study, PLA patterns having regular microstructured pillars with 12 different pillar diameters and pillar-to-pillar distances were prepared by hot pressing pre-flattened PLA sheets onto preformed polydimethylsiloxane (PDMS) soft molds having micro-sized pits. PDMS templates were previously prepared by photolithography using SU-8 molds. Apparent, advancing, and receding water contact angle measurements were carried out on the PLA patterns containing micropillars, and the morphology of the patterns was examined by optical and SEM microscopy. The largest contact angle obtained without the surface modification of the pure PLA pattern was 139°. Then, PLA micropatterns were hydrophobized using three types of silanes via chemical vapor deposition method, and SH-PLA patterns were obtained having θs of up to 167°. It was found that the highest θ values could be obtained when PLA pattern samples were coated with a silane containing a fluorine atom in its chemical structure. Washing and service life stability tests were also performed on the coated pattern samples and all of the silane coatings on the PLA patterns were found to be resistant over a 6 month period.

Polysiloxane Nanofilaments Infused with Silicone Oil Prevent Bacterial Adhesion and Suppress Thrombosis on Intranasal Splints.

Like all biofluid-contacting medical devices, intranasal splints are highly prone to bacterial adhesion and clot formation. Despite their widespread use and the numerous complications associated with infected splints, limited success has been achieved in advancing their safety and surface biocompatibility, and, to date, no surface-coating strategy has been proposed to simultaneously enhance the antithrombogenicity and bacterial repellency of intranasal splints. Herein, we report an efficient, highly stable lubricant-infused coating for intranasal splints to render their surfaces antithrombogenic and repellent toward bacterial cells. Lubricant-infused intranasal splints were prepared by creating superhydrophobic polysiloxane nanofilament (PSnF) coatings using surface-initiated polymerization of n-propyltrichlorosilane (n-PTCS) and further infiltrating them with a silicone oil lubricant. Compared with commercially available intranasal splints, lubricant-infused, PSnF-coated splints significantly attenuated plasma and blood clot formation and prevented bacterial adhesion and biofilm formation for up to 7 days, the typical duration for which intranasal splints are kept. We further demonstrated that the performance of our engineered biointerface is independent of the underlying substrate and could be used to enhance the hemocompatibility and repellency properties of other medical implants such as medical-grade catheters.

Practical Applications of Superhydrophobic Materials and Coatings: Problems and Perspectives.

Synthetic superhydrophobic (SH) surfaces were developed after 1990s, and the number of publications in this field is around 13 500 at present. However, the industrial production of SH coatings is very unsatisfying after the intensive research activity in the last two decades. The main reason is the loss of the water repellence properties when SH surfaces are exposed to outdoor conditions due to their weak mechanical properties and contamination from the medium which removes the initial SH properties. In this Feature Article, we focus on the scientific and technical reasons which prevent the application of the SH surfaces in our daily lives by highlighting some well-known but mostly overlooked problems in this area. (The synthesis methods of SH surfaces are not the subject of this article since they were reviewed previously in very good articles.) The basic contact angle science and the issue of the cancellation of the Wenzel and Cassie-Baxter equations are reviewed in the first part. The issues of the expensive and small-scale SH surface preparation problems, the difficulties in obtaining a transparent SH surface, the troubles arising from the water vapor condensation on an SH surface, the lack of robustness and abrasion resistance of most of the SH surfaces, the drawbacks of the fabricated self-healing SH surfaces, the short useful service life of self-cleaning SH surfaces due to surface contamination, and the ineffective anti-icing SH coatings are reviewed in the following text. Some important problems affecting the unsuccessful industrial applications of the SH surfaces are discussed critically in the Conclusions and Outlook section. Finally, some proposals are presented for future directions on the synthesis and applications of SH surfaces.

Simple Model for Diffusion-Limited Drop Evaporation of Binary Liquids from Physical Properties of the Components: Ethanol-Water Example.

The understanding of the evaporation process of drops consisting of binary mixtures, in particular ethanol-water drops, is important in many industries such as ink-jet printing, cooling of microelectronics, and alcohol-added pesticide spray applications. The theory of the diffusion-limited drop evaporation process for pure liquids has been investigated thoroughly, and linear (dV(2/3)/dt) slopes were obtained for most of the cases. However, the evaporation of binary liquid drops was found to be much more complicated than that of the pure liquids due to the change of the composition of the drop by time and there is a need for the development of a new model. The experimental results on the diffusion-limited drop evaporation behavior of ethanol-water binary drops initially containing 25 and 50% ethanol by wt and having a volume of 7 µL were reported on a flat hydrophobic Teflon-FEP substrate under the constant relative humidity of 54% and 25 °C temperature conditions, together with pure liquids. The change of contact angles, heights, and contact radius of the drops by time were monitored with a camera. In a parallel study, the concentration changes in the bulk composition of ethanol-water binary drops of 7 µL (25 and 50% ethanol by wt) by time in the same evaporation conditions were monitored using a refractive index-ethanol concentration calibration curve. Then, the parameters affecting the drop evaporation process, such as total vapor pressures, average diffusion coefficient of binary vapors, average molecular weights, and densities of the liquid drops, were calculated using well-known physical chemistry approaches from the previously published data. These parameters were used to estimate the rate of binary ethanol-water drop evaporation, and it was determined that the proposed model fitted the (dV(2/3)/dt) slopes obtained from experimental data points with lower than 5% error when the surface cooling of the drops was considered.

Wet-spun graphene filaments: effect of temperature of coagulation bath and type of reducing agents on mechanical & electrical properties.

Although many factors are considered to improve the properties of graphene filaments, there is no report in the existing literature on the effect of the temperature of the coagulation bath to the mechanical properties of graphene oxide filaments obtained in the wet-spinning process and also to the mechanical and electrical properties of the resulting graphene filaments after reduction. In this study, the effect of the temperature of the isopropanol coagulation bath during wet-spinning of graphene filaments on their final properties after formation was investigated and it was found that the decrease of the coagulation bath temperature resulted in more compact filaments having better mechanical properties for both graphene oxide and corresponding reduced graphene filaments. The best tensile strength and Young's modulus values were obtained in isopropanol coagulation bath which was kept at 15 °C. On the other hand, the types of the chemical reduction agents which can provide better electrical conductivity to graphene filaments after reduction were also investigated and it was determined that the use of hydriodic acid/acetic acid mixture resulted in graphene filaments having the best electrical conductivity (1.28 × 104 S m-1) and also tensile strength (234 ± 26 MPa) values. The addition of acetic acid into hydriodic acid increased the tensile strength 26% when compared with the plain HI treatment. Both electrical conductivity and tensile strength results were higher than most of the previously reported values of the wet-spun neat graphene filaments in the literature.

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.

Control of stain geometry by drop evaporation of surfactant containing dispersions.

Control of stain geometry by drop evaporation of surfactant containing dispersions is an important topic of interest because it plays a crucial role in many applications such as forming templates on solid surfaces, in ink-jet printing, spraying of pesticides, micro/nano material fabrication, thin film coatings, biochemical assays, deposition of DNA/RNA micro-arrays, and manufacture of novel optical and electronic materials. This paper presents a review of the published articles on the diffusive drop evaporation of pure liquids (water), the surfactant stains obtained from evaporating drops that do not contain dispersed particles and deposits obtained from drops containing polymer colloids and carbon based particles such as carbon nanotubes, graphite and fullerenes. Experimental results of specific systems and modeling attempts are discussed. This review also has some special subtopics such as suppression of coffee-rings by surfactant addition and "stick-slip" behavior of evaporating nanosuspension drops. In general, the drop evaporation process of a surfactant/particle/substrate system is very complex since dissolved surfactants adsorb on both the insoluble organic/inorganic micro/nanoparticles in the drop, on the air/solution interface and on the substrate surface in different extends. Meanwhile, surfactant adsorbed particles interact with the substrate giving a specific contact angle, and free surfactants create a solutal Marangoni flow in the drop which controls the location of the particle deposition together with the rate of evaporation. In some cases, the presence of a surfactant monolayer at the air/solution interface alters the rate of evaporation. At present, the magnitude of each effect cannot be predicted adequately in advance and consequently they should be carefully studied for any system in order to control the shape and size of the final deposit.

Osteoselection supported by phase separated polymer blend films.

The instability of implants after placement inside the body is one of the main obstacles to clinically succeed in periodontal and orthopedic applications. Adherence of fibroblasts instead of osteoblasts to implant surfaces usually results in formation of scar tissue and loss of the implant. Thus, selective bioadhesivity of osteoblasts is a desired characteristic for implant materials. In this study, we developed osteoselective and biofriendly polymeric thin films fabricated with a simple phase separation method using either homopolymers or various blends of homopolymers and copolymers. As adhesive and proliferative features of cells are highly dependent on the physicochemical properties of the surfaces, substrates with distinct chemical heterogeneity, wettability, and surface topography were developed and assessed for their osteoselective characteristics. Surface characterizations of the fabricated polymer thin films were performed with optical microscopy and SEM, their wettabilities were determined by contact angle measurements, and their surface roughness was measured by profilometry. Long-term adhesion behaviors of cells to polymer thin films were determined by F-actin staining of Saos-2 osteoblasts, and human gingival fibroblasts, HGFs, and their morphologies were observed by SEM imaging. The biocompatibility of the surfaces was also examined through cell viability assay. Our results showed that heterogeneous polypropylene polyethylene/polystyrene surfaces can govern Saos-2 and HGF attachment and organization. Selective adhesion of Saos-2 osteoblasts and inhibited adhesion of HGF cells were achieved on micro-structured and hydrophobic surfaces. This work paves the way for better control of cellular behaviors for adjustment of cell material interactions.

Selective adsorption of L1210 leukemia cells/human leukocytes on micropatterned surfaces prepared from polystyrene/polypropylene-polyethylene blends.

The objective of this study is to prepare polymeric surfaces which will adsorb L1210 leukemia cells selectively more than that of healthy human leukocytes in order to develop new treatment options for people with leukemia. Chemically heterogeneous and micropatterned surfaces were formed on round glass slides by dip coating with accompanying phase-separation process where only commercial polymers were used. Surface properties were determined by using optical microscopy, 3D profilometry, SEM and measuring contact angles. Polymer, solvent/nonsolvent types, blend composition and temperature were found to be effective in controlling the dimensions of surface microislands. MTT tests were applied for cell viability performance of these surfaces. Polystyrene/polyethylene-polypropylene blend surfaces were found to show considerable positive selectivity to L1210 leukemia cells where L1210/healthy leukocytes adsorption ratio approached to 9-fold in vitro. Effects of wettability, surface free energy, microisland size geometry on the adsorption performances of L1210/leukocytes pairs are discussed.

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.

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.

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.

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.

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.

Transformation of a simple plastic into a superhydrophobic surface.

Superhydrophobic surfaces are generally made by controlling the surface chemistry and surface roughness of various expensive materials, which are then applied by means of complex time-consuming processes. We describe a simple and inexpensive method for forming a superhydrophobic coating using polypropylene (a simple polymer) and a suitable selection of solvents and temperature to control the surface roughness. The resulting gel-like porous coating has a water contact angle of 160 degrees. The method can be applied to a variety of surfaces as long as the solvent mixture does not dissolve the underlying material.