Showing papers on "Sessile drop technique published in 2017"

Dynamics and universal scaling law in geometrically-controlled sessile drop evaporation

Abstract: The evaporation of a liquid drop on a solid substrate is a remarkably common phenomenon. Yet, the complexity of the underlying mechanisms has constrained previous studies to spherically symmetric configurations. Here we investigate well-defined, non-spherical evaporating drops of pure liquids and binary mixtures. We deduce a universal scaling law for the evaporation rate valid for any shape and demonstrate that more curved regions lead to preferential localized depositions in particle-laden drops. Furthermore, geometry induces well-defined flow structures within the drop that change according to the driving mechanism. In the case of binary mixtures, geometry dictates the spatial segregation of the more volatile component as it is depleted. Our results suggest that the drop geometry can be exploited to prescribe the particle deposition and evaporative dynamics of pure drops and the mixing characteristics of multicomponent drops, which may be of interest to a wide range of industrial and scientific applications.

Influence of surface energetics of graphene oxide on fracture toughness of epoxy nanocomposites

Abstract: The effects of the addition of graphene oxide (GO) as a filler for an epoxy matrix have been studied in terms of the surface energy and mechanical interfacial properties of GO/epoxy nanocomposites. The GO surface properties were determined using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The contact angle was measured by the sessile drop method for the evaluation of surface free energy. The investigated mechanical properties of the nanocomposites included the impact strength, fracture toughness and fracture energy. For the GO-reinforced epoxy resin matrix system, a direct linear relationship was observed between the specific polar components of the surface energy and the mechanical behavior. These results indicate that the mechanical interfacial properties of the GO/epoxy nanocomposites were controlled by the specific polar component including the electron–acceptor and electron–donor parameters.

Wetting and Spreading of Molten Volcanic Ash in Jet Engines

Abstract: A major hazard to jet engines posed by volcanic ash is linked to the wetting and spreading of molten ash droplets on engine component surfaces. Here, using the sessile drop method, we study the evolution of the wettability and spreading of volcanic ash. We employ rapid temperature changes up to 1040–1450 °C, to replicate the heating conditions experienced by volcanic ash entering an operating jet engine. In this scenario, samples densify as particles coalesce under surface tension until they form a large system-sized droplet (containing remnant gas bubbles and crystals), which subsequently spreads on the surface. The data exhibit a transition from a heterogeneous to a homogeneous wetting regime above 1315 °C as crystals in the drops are dissolved in the melt. We infer that both viscosity and microstructural evolution are key controls on the attainment of equilibrium in the wetting of molten volcanic ash droplets.

Shape of a sessile drop on a flat surface covered with a liquid film.

Abstract: Motivated by the development of lubricant-infused slippery surfaces, we study a sessile drop of a nonvolatile (ionic) liquid which is embedded in a slowly evaporating lubricant film (n-decane) on a horizontal, planar solid substrate. Using laser scanning confocal microscopy we imaged the evolution of the shape of the liquid/liquid and liquid/air interfaces, including the angles between them. Results are compared to solutions of the generalized Laplace equations describing the drop profile and the annular wetting ridge. For all film thicknesses, experimental results agree quantitatively with the calculated drop and film shapes. With the verified theory we can predict height and volume of the wetting ridge. Two regimes can be distinguished: for macroscopically thick films (excess lubrication) the meniscus size is insensitive to changes in film thickness. Once the film is thin enough that surface forces between the lubricant/air and solid/lubricant interfaces become significant the meniscus changes significantly with varying film thickness (starved lubrication). The size of the meniscus is particularly relevant because it affects sliding angles of drops on lubricant-infused surfaces.

Influence of cold plasma treatment parameters on the mechanical properties of polyamide homogeneous bonded joints

Abstract: This study reports a systematic investigation of the effects of various cold plasma treatment parameters on adhesion characteristics of two polyamide substrates. Polyamide 6 and polyamide 6.6 specimens were treated with a low pressure radio-frequency discharge plasma using different treatment time, power inputs and working gas (air, argon and oxygen). Contact angle measurements with sessile drop technique were carried out for estimation of surface wettability, as well as surface roughness evaluation and X-ray Photoelectron Spectroscopy (XPS) analysis. Then, untreated samples and cold plasma treated samples were adhesively bonded together to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to investigate the effect of different surface treatments on the joint strength. The experimental results show that the optimized plasma process may remarkably increase wettability properties of polyamide surface and shear strength of the bonded joints.

Tailoring crack morphology in coffee-ring deposits via substrate heating.

Abstract: The drying of a sessile drop consisting of colloidal particles and the formation of particulate deposits with spatially periodic cracks were ubiquitous. The drying induced stress, which is generated during the evaporation of a colloidal drop, is released by the formation of cracks. We find that the morphology of cracks formed in particulate films dried at substrate temperature, Tsub = 25 °C is markedly different from that of cracks formed at Tsub > 45 °C. The cracks are disordered in the former case, but ordered and periodic in the latter. The disorderedness of cracks observed at Tsub = 25 °C is mainly due to the formation of a coffee-ring like particle deposit that exhibits a larger height gradient. The ultimate deposit pattern after complete drying is observed to be different for colloidal dispersion drops evaporated at different substrate temperatures. This is attributed to temperature-dependent solvent flow mechanisms and capillary-driven flow, which occur inside the colloidal drop during the course of drying. In addition, for the coffee-ring-like particulate deposit obtained at Tsub ≤ 45 °C, the ratio between the width of the deposit w and the radius of the ring R scales with the volume fraction of the colloids φ, w/R ∼ φ0.5, in the range of volume fractions studied in this work. The deposited patterns obtained at temperature Tsub > 45 °C are largely dominated by the capture of particles by the receding liquid–vapor interface. This is due to the faster rate of decrease of the liquid–vapor interface position with an increase in substrate temperature.

Vacuum Brazing of WC-8Co Cemented Carbides to Carbon Steel Using Pure Cu and Ag-28Cu as Filler Metal

Abstract: The wetting and spreading behavior of commercial pure Cu and Ag-28Cu alloy on WC-8Co cemented carbide were investigated by the sessile drop technique. The contact angle of both systems obviously decreases with moderately increasing the wetting temperature. Vacuum brazing of the WC-8Co cemented carbide to SAE1045 steel using the pure Cu or Ag-28Cu as filler metal was further carried out based on the wetting results. The interfacial interactions and joint mechanical behavior involving microhardness, shear strength and fracture were analyzed and discussed. An obvious Fe-Cu-Co transition layer is detected at the WC-8Co/Cu interface, while no obvious reaction layer is observed at the whole WC-8Co/Ag-28Cu/SAE1045 brazing seam. The microhardness values of the two interlayers and the steel substrate near the two interlayers increase more or less, while those of WC-8Co cemented carbide substrates adjacent to the two interlayers decrease. The WC-8Co/SAE1045 joints using pure Cu and Ag-28Cu alloy as filler metals obtain average shear strength values of about 172 and 136 MPa, and both of the joint fractures occur in the interlayers.

Capillary Drop Penetration Method to Characterize the Liquid Wetting of Powders.

Abstract: We present a method to characterize the wettability of powders, based on the penetration dynamics of a sessile drop deposited on a slightly compressed powder bed. First, we show that a direct comparison of the wetting properties of different liquids is possible without having to solve the three-dimensional liquid penetration problem, by considering the appropriate dimensionless variables. We show that the contact area between the sessile drop and the powder bed remains constant during most of the penetration process and demonstrate that as a result, the evolution of the dimensionless penetration volume is given by a universal function of the dimensionless time, with no dimensionless parameters. Then, using a reference liquid that completely wets the powder, it is possible to obtain an effective contact angle for a test liquid of interest, independent of other properties of the powder bed, such as permeability and a characteristic pore size. We apply the proposed method to estimate the contact angle of water with different powder blends, by using silicone oil as the reference liquid. Finally, to highlight the potential of the proposed method to characterize pharmaceutical powders, we consider a blend of lactose, acetaminophen, and a small amount of lubricant (magnesium stearate). The proposed method adequately captures a significant decrease in hydrophilicity that results from exposing the blend to excessive mixing, a well-known effect in the pharmaceutical industry.

Understanding desiccation patterns of blood sessile drops.

Abstract: Desiccation of a blood sessile drop on a glass surface leads to the formation of interesting cracking patterns. These desiccation patterns have been identified to have three characteristic regions, i.e., peripheral, coronal and central regions. Driving forces responsible for the formation of cracking patterns are the redistribution of colloidal materials driven by a “coffee ring” effect and the time- and location-dependent development of internal stresses caused by water evaporation and progressive gelation from the drop edge to its center. Since the concentrations of colloidal materials, i.e., cellular components, protein macromolecules and other constituents (glucose, bilirubin and lipids) in blood, influence the cracking patterns, an understanding of these patterns can potentially reveal clues for the evaluation of health conditions and offer a low-cost diagnostic tool for human diseases. This study presents a mechanistic analysis of the pattern formation in desiccating blood sessile drops. We focus on the build-up and release of internal stresses by examining the cracking process. Optical and scanning electron microscopes (SEM) were used to capture the initiation, propagation and directions of cracks in different regions. The propagation and widening of orthoradial and radial cracks in relation to the adhesion and cohesion of the blood sessile drops were observed and characterized. New microscopic insights into internal stress releasing processes provide a new understanding of physical events occurring underneath the gelled film of the blood sessile drop and differences in the distribution of strain energy in different regions, which will aid our understanding of different cracking patterns in those regions.

Direct Measurement of Static and Dynamic Contact Angles Using a Random Micromodel Considering Geological CO2 Sequestration

Abstract: The pore-level two-phase fluids flow mechanism needs to be understood for geological CO2 sequestration as a solution to mitigate anthropogenic emission of carbon dioxide. Capillary pressure at the interface of water–CO2 influences CO2 injectability, capacity, and safety of the storage system. Wettability usually measured by contact angle is always a major uncertainty source among important parameters affecting capillary pressure. The contact angle is mostly determined on a flat surface as a representative of the rock surface. However, a simple and precise method for determining in situ contact angle at pore-scale is needed to simulate fluids flow in porous media. Recent progresses in X-ray tomography technique has provided a robust way to measure in situ contact angle of rocks. However, slow imaging and complicated image processing make it impossible to measure dynamic contact angle. In the present paper, a series of static and dynamic contact angles as well as contact angles on flat surface were measured inside a micromodel with random pattern of channels under high pressure condition. Our results showed a wide range of pore-scale contact angles, implying complexity of the pore-scale contact angle even in a highly smooth and chemically homogenous glass micromodel. Receding contact angle (RCA) showed more reproducibility compared to advancing contact angle (ACA) and static contact angle (SCA) for repeating tests and during both drainage and imbibition. With decreasing pore size, RCA was increased. The hysteresis of the dynamic contact angle (ACA–RCA) was higher at pressure of one megapascal in comparison with that at eight megapascals. The CO2 bubble had higher mobility at higher depths due to lower hysteresis which is unfavorable. CO2 bubbles resting on the flat surface of the micromodel channel showed a wide range of contact angles. They were much higher than reported contact angle values observed with sessile drop or captive bubble tests on a flat plate of glass in previous reports. This implies that more precaution is required when estimating capillary pressure and leakage risk.

Adhesion evaluation of asphalt-aggregate interface using surface free energy method

Abstract: The influence of organic additives (Sasobit and RH) and water on the adhesion of the asphalt-aggregate interface was studied according to the surface free energy theory. Two asphalt binders (SK-70 and SK-90), and two aggregate types (limestone and basalt) were used in this study. The sessile drop method was employed to test surface free energy components of asphalt, organic additives and aggregates. The adhesion models of the asphalt-aggregate interface in dry and wet conditions were established, and the adhesion work was calculated subsequently. The energy ratios were built to evaluate the effect of organic additives and water on the adhesiveness of the asphalt-aggregate interface. The results indicate that the addition of organic additives can enhance the adhesion of the asphalt-aggregate interface in dry conditions, because organic additives reduced the surface free energy of asphalt. However, the organic additives have hydrophobic characteristics and are sensitive to water. As a result, the adhesiveness of the asphalt-aggregate interface of the asphalt containing organic additives in wet conditions sharply decreased due to water damage to asphalt and organic additives. Furthermore, the compatibility of asphalt, aggregate with organic additive was noted and discussed.

Stability of a liquid bridge between nonparallel hydrophilic surfaces

Abstract: Formation of liquid bridges between two solid surfaces is frequently observed in industry and nature, e.g. in printing applications. When the two solid surfaces are not parallel (with a dihedral angle ψ between them), an interesting phenomenon emerges: if ψ exceeds a critical angle (denoted as ψc) the bridge is no longer stable, and propels itself toward the cusp of the surfaces. In this work we performed, for the first time, a systematic study on the parameters influencing ψc by combining experimental, theoretical, and numerical investigations. It was shown that ψc is determined by the advancing contact angle (θa) and Contact Angle Hysteresis (CAH) of the surfaces: it increases as θa or CAH increases, and these two parameters have a nonlinear and interdependent influence on ψc. Based on our quantitative results, an empirical equation is presented to predict the critical angle, ψc=f(θa,CAH) in closed analytical form. This equation can be used to calculate ψc for bridges formed by moving down a pre-tilted surface towards a sessile drop on a stationary lower surface, or bridges between initially parallel surfaces which the top surface tilts after bridge formation.

Wettability phenomena of molten steel in contact with alumina substrates with alumina and alumina-carbon coatings

Abstract: The effects of the surface roughness, carbon content and the additions of carbon nanotubes (CNT) and alumina nanosheets (ANS) on the wettability of molten steel on Al2O3 substrates with and without Al2O3(-C) coatings were investigated by the sessile drop wetting method, SEM and EDS, etc. At the beginning of the wetting experiment, the contact angle increased with an increasing surface roughness. With a longer wetting time, the interfacial reactions between the coatings and molten steel became more influential on the contact angles. When the coatings contained 30 wt% carbon, a new layer presumably consisting of hercynite (FeAl2O4) was formed at the substrate/steel interface stabilizing the contact angles with time. Moreover, compared to the coatings without nano-additives, the additions of CNT and ANS promoted the reactions at lower temperatures and resulted in a faster stabilization of the contact angles. Finally, the interfacial wetting and reaction mechanisms were proposed.

Calculation of the Contact Angle of Polymer Melts on Tool Surfaces from Viscosity Parameters

Abstract: It is of great importance for polymer processing whether and how viscosity influences the wettability of tool surfaces. We demonstrate the existence of a distinct relationship between the contact angle of molten polymers and zero shear viscosity in this paper. The contact angle of molten polypropylene and polymethylmethacrylate on polished steel was studied in a high temperature chamber using the sessile drop method. A high pressure capillary rheometer with a slit die was employed to determine the shear viscosity curves in a low shear rate range. A linear relation between the contact angle and zero shear viscosity was obtained. Furthermore, the contact angle and the zero shear viscosity values of the different polymers were combined to one function. It is revealed that, for the wetting of tool surfaces by molten polymers, a lower viscosity is advantageous. Furthermore, a model based on the temperature shift concept is proposed which allows the calculation of the contact angle of molten polymers on steel for different temperatures directly from shear viscosity data.