Sessile Water Drop Research Articles

The sessile drop work of adhesion revisited

Work of adhesion per unit area of the interface to separate two distinct phases, and creating new interfaces. For the case of sessile drop, the Young-Dupre equation is usually used to express the work of adhesion, but it assumes the newly formed drop preserves its shape after detachment, which is not correct. Also, initial and final drop shapes cannot provide the work of adhesion. We argued that the work of adhesion depends on the path (despite the internal energy), and suggest a methodology to calculate the work of adhesion, based on calculating the mechanical work during the detachment, i.e. integration of drop centroid displacement times the external resistive forces. To put things into perspective, a 4 μl sessile water drop was studied with initial (advancing) and receding contact angles of 79.7° ± 0.9°, 60.2° ± 0.8°, respectively. The direct calculation of work of adhesion using the developed methodology in this paper gives 0.03 J/m2, while Young-Dupre gives 0.085 J/m2, and calculating the work of adhesion based on initial (sessile) and final (full sphere) states of the drop gives a negative value, i.e. −1.49 J/m2. So, neither Young-Dupre relation nor the initial and final energy states of drop can be used to calculate the work of adhesion, and detachment path should be considered in the calculation of work of adhesion.

Surface adhesion and physical properties of modified TPS and PBAT multilayer film

Starch-based materials are susceptible to moisture, leading to sticking and instability. Coating with hydrophobic biodegradable materials improves their stability. This research produced multilayer biodegradable packaging based on thermoplastic cassava starch (TPS) blended with poly(butylene adipate-co-terephthalate) (PBAT). TPS/PBAT mixtures were produced via blown film extrusion before coating with PBAT solution. Different TPS content (up to 80 %) and types of modified cassava starch namely native starch (NS), hydroxypropylated starch (HS), acetylated starch (AS), and octenyl-succinated starch (OS) were investigated for micro and chemical structure, delamination, and PBAT coating deposition on TPS/PBAT multilayer film. IR spectra of the peak at 3300 cm−1 (O-H stretching of TPS) was absent after coating, suggesting reduced TPS content on the film surface, while 1717 cm−1 (CO from carbonyl groups of PBAT), became sharper, indicating that PBAT deposition covered the film surface. The X-ray diffractogram showed a 2θ peak located at 24.4° (starch crystallinity) in the coated NS film; however, the peak was absent in coated HS, AS, and OS films because bulky chemical-grafted modified starch prevented molecular arrangement. Hydrophilic NS showed delamination and film substrate swelling after 60 s of direct contact with a sessile drop of water, indicating poor affinity. Hydrophobic-modified AS and OS significantly improved affinity with the PBAT coating layer giving a higher contact angle (105–112°), preventing delamination and substrate swelling and indicating increased water resistance. AS and OS (60 %) had higher shrinkage (23.78 % and 47.33 %) and a thicker coating layer (11.51 and 10.35 µm) due to greater polymer-solvent interaction. Hydrophobic-modified starch successfully improved the interaction between film layers and increased PBAT deposition.

Polarity-Induced Reactive Wetting: Spreading and Retracting Sessile Water Drops.

Wetting is typically defined by the relative liquid to solid surface tension/energy, which are composed of polar and nonpolar subcontributions. Current studies often assume that they remain invariant, that is, surfaces are wetting-inert. Complex wetting scenarios, such as adaptive or reactive wetting processes, may involve time-dependent variations in interfacial energies. To maximize differences in energetic states, we employ low-energy perfluoroalkyls integrated with high-energy silica-based polar moieties grown on low-energy polydimethylsiloxane. To this end, we tune the hydrophilic-like wettability on these perfluoroalkyl-silica-polydimethylsiloxane surfaces. Drop contact behaviors range from invariantly hydrophobic at ca. 110° to rapidly spreading at ca. 0° within 5 s. Unintuitively, these vapor-grown surfaces transit toward greater hydrophilicity with increasing perfluoroalkyl deposition. Notably, this occurs as sequential silica-and-perfluoroalkyl deposition also leaves behind embedded polar moieties. We highlight how surfaces having such chemical heterogeneity are inherently wetting-reactive. By creating an abrupt wetting transition composed of reactive and inert domains, we introduce spatial dependency. Drops contacting the transition spread before retracting, occurring over the time scale of a few seconds. This phenomenon contradicts current understanding, exhibiting a uniquely (1) decreasing advancing contact angle and (2) increasing receding contact angle. To explain the behavior, we model such time- and space- dependent reactive wetting using first order kinetics. In doing so, we explore how reactive and recovery mechanisms govern the characteristic time scales of spreading and retracting sessile drops.

Different Methods to Modify the Hydrophilicity of Titanium Implants with Biomimetic Surface Topography to Induce Variable Responses in Bone Marrow Stromal Cells.

The osteoblastic differentiation of bone marrow stromal cells (bMSCs), critical to the osseointegration of titanium implants, is enhanced on titanium surfaces with biomimetic topography, and this is further enhanced when the surfaces are hydrophilic. This is a result of changing the surface free energy to change protein adsorption, improving cell attachment and differentiation, and improving bone-to-implant contact in patients. In this study, we examined different methods of plasma treatment, a well-accepted method of increasing hydrophilicity, and evaluated changes in surface properties as well as the response of bMSCs in vitro. Commercially pure Ti and titanium-aluminum-vanadium (Ti6Al4V) disks were sand-blasted and acid-etched to impart microscale and nanoscale roughness, followed by treatment with various post-processing surface modification methods, including ultraviolet light (UV), dielectric barrier discharge (DBD)-generated plasma, and plasma treatment under an argon or oxygen atmosphere. Surface wettability was based on a sessile water drop measurement of contact angle; the elemental composition was analyzed using XPS, and changes in topography were characterized using scanning electron microscopy (SEM) and confocal imaging. The cell response was evaluated using bMSCs; outcome measures included the production of osteogenic markers, paracrine signaling factors, and immunomodulatory cytokines. All plasma treatments were effective in inducing superhydrophilic surfaces. Small but significant increases in surface roughness were observed following UV, DBD and argon plasma treatment. No other modifications to surface topography were noted. However, the relative composition of Ti, O, and C varied with the treatment method. The cell response to these hydrophilic surfaces depended on the plasma treatment method used. DBD plasma treatment significantly enhanced the osteogenic response of the bMSCs. In contrast, the bMSC response to argon plasma-treated surfaces was varied, with an increase in OPG production but a decrease in OCN production. These results indicate that post-packaging methods that increased hydrophilicity as measured by contact angle did not change the surface free energy in the same way, and accordingly, cells responded differently. Wettability and surface chemistry alone are not enough to declare whether an implant has an improved osteogenic effect and do not fully explain how surface free energy affects cell response.

Bilayer nonwovens using natural rubber, poly(lactic acid) and bactericidal nanoparticles for wound dressings

Wound healing poses a significant challenge in healthcare, impacting not only on patients' quality of life but also placing a substantial financial burden on healthcare systems. Bilayer nonwovens have emerged as a promising solution for wound dressings due to their porous morphology, providing a high surface area for cellular adhesion and growth, and their permeability, allowing proper oxygen and fluid exchange to promote wound healing. When bactericide materials are added to the formulation, they can decrease or prevent infection caused by microorganisms, further aiding the healing process. Solution blow spinning is an appealing technique for creating bilayer nonwovens as it can produce high yields at a low cost. Yet, even though natural rubber boasts a well-established safety record in biomaterials, it remains an underexplored candidate for generating micro-nanofibrous structures. In this study, solution blow spinning was employed to fabricate bilayer composite nonwovens for wound dressing materials. The material architecture consisted of a bottom layer composed of a blend of natural rubber, poly(lactic acid) (PLA), and polyethylene glycol in the respective proportion of 2:1:3 w/w and a top layer composed of PLA and ZnO nanoparticles added in concentrations of 1%, 2.5%, and 5% based on the PLA mass of this layer. The top layer was further functionalized with Ag nanoparticles through spraying. The obtained materials demonstrated satisfactory hydrophobicity for cellular adhesion in the bottom layer, ranging from 51.95° to 92.20° of water sessile drop contact angle, and tunable mechanical strength, which could be controlled by varying the content of ZnO nanoparticles. Additionally, the material showed antimicrobial properties due to the presence of added Ag nanoparticles, able to inhibit E. coli, S. enterica serovar Typhimurium, and S. aureus by direct contact. Besides, the nonwoven prevented the passage of microorganisms due to the pore size of the structure. Overall, the unique properties of the developed material, such as its low cost, ease of production, and remarkable wound-healing properties, make it a promising candidate for wound dressing applications.

Hydrate Growth over a Sessile Drop of Water in Cyclopentane

Liquid cyclopentane is frequently used in hydrate formation studies as an analogue of natural gas because cyclopentane hydrates are stable above the ice melting point at ambient pressure. In this study, hydrate growth was established on a sessile water drop of 11 mm in diameter and 4.5 mm in height (volume of 0.25 mL) immersed in liquid cyclopentane. The hydrate formation mechanism and growth processes were observed optically over an extended range of subcooling temperatures from 5.1 to 15.2 °C, with the cyclopentane bulk temperature maintained in different experimental runs between 2.6 and −7.5 °C. Qualitative and quantitative comparisons were performed to confirm the absence of ice freezing during hydrate formation, and thus, the lack of contamination of the latter from the former in the experiments. Different transformations in the hydrate film morphology were registered from macroscopic observation over the considered range of subcooling temperatures, with the hydrate crystals composing the film taking the form of polyhedral, dendritic, or spherulitic structures. It was also found that the hydrate growth rate varied depending on the subcooling temperature, with the variation of the growth rate as a function of this temperature changing from a power to an (approximately) linear law with an increase in the degree of subcooling. We postulate that hydrate film growth can be governed by different mechanisms, whose roles change over the range of explored subcooling temperatures.

Rainbows in Different Refractive Indices

The rainbow is a natural optical scattering and dispersion phenomenon that reveals the visible spectral composition of sunlight in the shape of an arc. People are instinctively attracted by its colorful appearance and curved shape. Hence, there are many serious studies about the rainbow with a long history. Recently, several simple experiments, adopting glass balls, acrylic spheres, spherical flasks, or sessile water drops, have been devised to demonstrate how the rainbow is formed. These works demonstrate the colors and shapes of the rainbow well and explain how the dispersive spectrum is produced by the refraction–reflection–refraction process. However, the influence of the refractive index is rarely illustrated. It is not difficult to see that the refractive index of raindrops and the atmosphere is closely related to the rainbow, especially the viewing angle of it. In this paper, we use spherical lenses with different materials and in different solutions to change the refractive index. Under a collimated light source, the evolution of the viewing angles of primary and secondary rainbows with respect to the refractive index is demonstrated. Experiments with refraction conditions similar to a natural rainbow are also conducted.

Surface modifications of a silicalite film designed for coating orthopaedic implants

The effect of various surface modifications of silicalite-1 film grown on a Ti6Al4V alloy on biocompatibility and cell differentiation has been addressed. The as-synthesized silicalite film was first modified by alkaline etching and by subsequent treatments in oxygen radiofrequency (O2 RF)plasma, by calcination at 500 °C, and by processing the calcinated samples in the O2 RFplasma. The chemical composition, crystallinity, topography and wettability of samples were characterized using angle-resolved X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, and sessile water drop contact angle measurements. We have discovered the presence of a non-zeolitic defective SiOx phase (NZP) with coordinated hydrocarbon chains. The NZP can be removed by the exposition to the O2 RF plasma. The metabolic activity tests done on human osteoblast line Saos-2 revealed that the NZP can influence cell proliferation; however, all studied modifications of silicalite film can be considered biocompatible. The osteogenic differentiation was studied on human mesenchymal stem cells under static conditions and under dynamic (pulsatile and shear) stress. The results obtained by the real-time qPCR and immunofluorescence microscopy suggest that etched, plasma-treated and calcinated + plasma-treated samples are the most promising for osteogenic cell differentiation.

Teflon graft poly N–N-dimethylacrylamide as a ciprofloxacin release system

This work presents the modification of Teflon films, formally named polytetrafluoroethylene, with dimethylacrylamide (DMA) through a graft copolymerization reaction initiated by gamma radiation (from Co-60) to endow the material with the capability to deliver hydrophilic drugs. Results suggest the surface of the grafted films is suitable for the load/release of ciprofloxacin, a broad-spectrum antibiotic, achieving the goal of the study. The analysis was supported by water sessile drop contact angles and swelling in water and phosphate buffers. Characterization of grafted films was completed by attenuated total reflectance infrared spectroscopy (FTIR-ATR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC).

Water infiltration into subcritical water-repellent soils with time-dependent contact angle

Soil water repellency, defined as the situation in which the affinity of soils to water is reduced, has been reported for many natural and agricultural soils worldwide. Soil water repellency has significant impacts on hydrology and geomorphology. It has been widely observed that the contact angle (CA) of a sessile water drop placed on a single-layer surface of water-repellent soil particles decreases in an exponential manner with time. This time-dependent CA has a substantial effect on flow in water-repellent soils. However, mathematical models aimed at modeling water flow in water-repellent soils disregard the time-dependent CA's effect. The current study aims to correct this omission. Using capillary rise of water with a time-dependent CA in a capillary tube, a model for water infiltration into subcritical water-repellent soils (CA < 90°) was developed. This model was successfully verified by performing infiltration experiments into (packed) coated glass beads. The CA at the wetting front during infiltration decreased exponentially to equilibrium, and the time to equilibrium was similar to that measured for a reference single layer of coated glass beads. Simulations for subcritical water-repellent soils with time-dependent CA yielded an infiltration rate that is either increasing with time (noted as concave) or initially increases and afterward decreases (noted as a concave-convex pattern).

Impact of a small disk on a sessile water drop

We address the details of events that follow the impact of a small solid disk on a water drop sitting on another disk of the same diameter. We experimentally demonstrate that fast squeezing of a low-viscosity liquid drop between two approaching disks leads to the formation of complex, radially expanding liquid structures (splashes) outside of the disks. The spatial and temporal dynamics of these splashes are tracked via high-speed video recording and flash photography. We analyze the mechanisms that control the shapes and breakup processes of these structures and derive a mathematical model for their behavior using simple physical arguments. Our investigation indicates that liquid structure formation is the result of a rapid increase in the velocity of liquid ejection from the gap between disks with time.

The influence of surface structuring on evaporation of sessile drops of water

The paper presents the results of experimental studies of the evaporation of sessile water droplets on smooth and structured surfaces of Teflon fluoropolymer. Using high-speed microphotographs, data were obtained on the change in the contact angle and the diameter of the contact spot of water droplets during evaporation on smooth and structured surfaces. Using the method of infrared thermography, the dynamics of changes in the average temperature of water droplets during evaporation was investigated. The results show that the structure of the surface has a decisive influence on the change in the geometric parameters and temperature of the sessile water droplets.

Convection in a droplet blown by gas flow

The behavior of convection in a sessile and suspended water drops is considered. Evaporation of spray droplets can be divided into two stages: evaporation in a gas flow and that on the wall. Usually, in the calculation of heat transfer and evaporation, liquid convection is neglected, which can lead to a significant error in the calculation. Therefore, there is a need in simple evaluation formulas to describe of the convection. The ratio of several key factors: the velocity of the forced gas flow, the thickness of the wall boundary layer, the thermogravitation convection, and the influence of thermal and concentration factors, is considered. For the first time it is shown that the convection velocity in a sessile drop is 30–40% less than in a suspended drop. The ratio of the maximum convection velocity to the average velocity is three for both the suspended and the sessile droplet. For a sessile drop, the effect of the solutal Marangoni flow on convection is much stronger than in a suspended drop. The obtained results and dependences are important for evaluating the role of convection in droplets evaporation in the spray, as well as for sessile droplets on a hot wall.

Size and Shape-Dependent Solubility of CuO Nanostructures.

In our theoretical study, the enhanced solubility of CuO nanoparticles in water saturated by air is predicted based on a simple thermodynamic model. CuO is considered in the form of nanoparticles with various shapes. The interfacial energy of a solid CuO/dilute aqueous solution interface was assessed by applying the average CuO surface energy and contact angle of a sessile drop of water. The equilibrium CuO solubility was calculated using Gibbs energy minimization technique. For the smallest spherical nanoparticles considered in this work (r = 2 nm), the solubility is significantly higher than the solubility of bulk material. In the case of cylindrical nanoparticles, the solubility increase is even more considerable. The CuO spherical nanoparticles solubility was also calculated using the Ostwald–Freundlich equation which is known to overestimate the solubility as discussed in this contribution.

Retraction: Bacterial adhesion and colonization differences between zirconia and titanium implant abutments: an in vivo human study.

[This retracts the article on p. 217 in vol. 42, PMID: 23346465.].

Spontaneous water adsorption-desorption oscillations in mesoporous thin films

Understanding fluid transport and phase changes in nanopore structures is of great interest to many application fields, from energy conversion to water harvesting. This work discusses the spontaneous oscillations of the water saturation of mesoporous thin films, in the zone adjacent to a sessile water drop, at ambient conditions. The wetting-front dynamics onto the film is described by considering three coexisting phenomena: infiltration from the water drop, condensation from air vapor, and evaporation to the ambient. It was found that the oscillations follow spontaneous condensation-evaporation imbalances, which are governed by the hysteretic character of the adsorption-desorption behavior of the mesoporous material. The outcomes of this work provide insights on the complex interplay between water and nanopore structures, which has practical implications for the handling of humid microenvironments in lab-on-a-chip technology, as well as for many processes that take part of the cycle of water in nature.

Effect of ZnO nanosizing on its solubility in aqueous media

Zinc pollution represents a great environmental risk, particularly with regards to the aquatic environment. In this theoretical contribution, the enhanced solubility of ZnO nanoparticles in pure water is predicted based on a simple thermodynamic model. The study used Zn2+, Zn(OH)+, Zn(OH)2, Zn(OH)3 − and Zn(OH)4 2− as dominant species in aqueous solution and a Gibbs energy minimisation method to calculate equilibrium Zn content in this solution. ZnO was investigated in the form of nanoparticles of various shapes, whose very high surface-to-volume ratio implicates their lower thermodynamic stability compared with bulk material. The interfacial energy of the solid ZnO – dilute aqueous solution interface was assessed by applying the average ZnO surface energy and contact angle of a sessile drop of water on ZnO()-O surface. At 298 K, the ratio of 2 nm spherical ZnO particles to the bulk material solubility was about 23.7. The calculated results were compared with experimental data and yielded a good agreement. These results are not only of great importance for nanomaterials research but they also have implications for environmental protection.

The hydrophobic surface state of talc as influenced by aluminum substitution in the tetrahedral layer

Talc is both an important industrial mineral product recovered by flotation, and also in other cases, a gangue mineral of concern in the flotation of certain sulfide ores, such as the PGM ores from South Africa and from the United States. The talc face surface is naturally hydrophobic with a water sessile drop contact angle of nearly 80°, which accounts for its flotation recovery in one case, and its contamination of sulfide mineral concentrates in other instances. Due to the presence of impurities in the talc structure the surface properties change. One such effect is the presence of aluminum, which can replace silicon in the silica tetrahedral layer of the talc structure. This results in a charge imbalance on the face surface because Si+4 is replaced by Al+3. Sessile drop contact angle and bubble attachment time measurements were made, and these results were compared to the results from molecular dynamics simulations (MDS). The extent of aluminum substitution in the silica tetrahedral layer was considered, and the sessile drop contact angle was found to decrease with increased aluminum content, decreasing from about 80° for no substitution (talc) to 0° for extensive substitution (phlogopite). The water film was found to be stable at the surface of highly aluminum substituted crystals due to the interaction between water molecules and the increased polarity of the surface state. This stable water film restricts the air bubble from attaching to such face surfaces. However, in the absence of aluminum substitution, no interactions between the water molecules and the face surface were observed and the air bubble readily attached to the face surface. This study provides additional understanding of how aluminum substitution in the tetrahedral layer affects the fundamental surface properties of talc, paving the way for the design of improved reagents for talc flotation as an industrial mineral product, and for talc depression in the recovery of sulfide mineral concentrates.

Letter: Dome-shaped splashes generated by the impact of a small disk on a sessile water drop

We investigate experimentally the splashes arising from the impact of a small solid disk on a water drop resting on another disk of the same diameter. The impact leads to an impulse radial ejection of water from the gap between the rapidly approaching disks. We revealed that the splashes have a complex spatial shape, which is a consequence of increasing liquid ejection velocity with time.

Clay Nanoparticles Composite Membranes Prepared with Three Different Polymers: Performance Evaluation

This paper presents the results obtained from the evaluation of clay nanoparticles as an additive for improving the characteristics and performance of composite membranes cast with polysulfone (PS), polyethersulfone (PES), and polyvinylidene fluoride (PVDF). Different concentrations of clay nanoparticles, ranging from 1 to 10% based on the polymer mass, were used to prepare all dope solutions. The addition of clay nanoparticles changed the internal pore morphology of membranes, which resulted in significant changes on their performance, regarding its water permeability, and fouling potential. The optimum nanoclay concentration for permeability enhancement was different for each polymer, 1.5%, 2.0%, and 6.0% for PS, PES, and PVDF, respectively. This difference can be attributed to the differences of polymer’s hydrophobicity, based on the contact angle of a sessile water drop, which is higher for PVDF (PVDF is more hydrophobic than PS and PES). The flow improvement changed based on the main polymer. Significant changes in internal pore structure were observed for all membranes. The proportion of macrovoids was decreased and pores had a better connectivity across the cross section for PES and PS membranes. For PVDF membranes, the addition of nanoclay had a different effect on their microstructure. In this case, internal pores were 20% wider, factor that increased the average membrane porosity. The simultaneous evaluation of the clay nanoparticles used as an additive have clearly demonstrated its potential application for composite membrane production. It is also worth to note that the best way for identifying and evaluating the potential for an additive for membrane casting is considering its effects for different polymers, under the same casting conditions.