Sessile drop technique

About: Sessile drop technique is a research topic. Over the lifetime, 2827 publications have been published within this topic receiving 68943 citations.

Self‐Assembly of Single‐Walled Carbon Nanotubes into a Sheet by Drop Drying

Abstract: Single-walled carbon nanotubes (SWNTs) are currently the focus of extensive interdisciplinary studies because of their unique physical and chemical properties and potential electronic applications, for example, in making sensors and fieldemission devices. Processing of SWNT-based materials into engineered macroscopic materials is still in its infancy; the most successful methods so far have been based on adapting techniques that had been developed in other areas of material science such as colloids and polymers. Recent successes include preparing fibers and ribbons of SWNTs; films of pure SWNTs, polymers doped with SWNTs, and growth in situ of SWNT arrays. Evaporation of drops on substrates has been used for patterned deposition of solutes onto non-porous substrates, such as in DNA microarrays, nanolithography, protein crystallization, and stretching DNA for hybridization studies. Shimoda et al. prepared continuous selfassembled films of SWNT bundles on glass near a receding contact line by solvent evaporation. The moving contact line of a drying drop could be similarly used to form aligned patterns of SWNTs on substrates for making films or for nanofabrication. Drops of a solution on a substrate follow one of two drying mechanisms: either the drop maintains a constant contact angle by de-pinning the contact line (e.g., water on non-wetting substrates), or the contact line gets pinned and the drop maintains a fixed contact area (e.g., colloidal dispersions). Deegan and co-workers have studied the drying of drops of colloidal dispersions and found that the particles deposit in a ring at the periphery of the drop due to capillary flow in which the pinned contact line causes the solvent to flow towards the edge. Recent investigations have also shown the formation of a skin or crust at the free surface of drops of polymers and colloidal suspensions. Pauchard and Allain found that the crust may collapse and evolve into different shapes as the surface area remains constant while the drop volume decreases due to solvent evaporation. “Crusting” on the surface of spin-cast films is a well-known phenomenon. De Gennes suggested a transport model for crust formation in spin-cast films. Because the glass-transition or gelation temperature of a pure polymer/colloid is higher than that in solution, at any temperature below the glass transition there is a critical particle concentration at which the system transitions from fluid to glassy or gel-like. Evaporation of solvent from the free surface leads to a local increase in concentration of the polymer/suspension at the free surface, and a very thin glassy or gelled crust is formed at the free surface. Here, we investigated drying of a sessile drop of individually suspended SWNTs in an aqueous solution of F68 Pluronic. We found that, instead of assembling on the substrate, the SWNTs selfassemble into a crust at the free surface. This entangled meshlike crust was characterized by various microscopy techniques. The “crusting” phenomenon could be used as a potential route for making thin coatings and films of SWNTs. Video microscopy showed that the initial drying progressed by de-pinning of the contact line, i.e., the radius of the base decreased with time. Figure 1a shows the drop radius (normalized by the initial radius) as a function of time. The diameter of each drop was measured at four different angles and then averaged. After about 360 s the drop attained a fixed base radius and a foot started appearing. Drops of pure water on the same substrate dried by maintaining a fixed base radius, in agreement with the findings by Birdi et al. Assuming quasistationary conditions, if diffusion of water in air is rate controlling, then in a sessile drop receding with a constant contact angle the square of the base radius is linear with time. While the drop radius shrunk, the contact angle between the drop and the glass substrate was about 10–15 °C (inferred by video microscopy). Up to t ∼ 210 s, we find that the assumption that diffusion is rate controlling is fairly accurate (Fig. 1b). We recorded the variation of weight of the drop with time (Fig. 1c) and found that the initial evaporation rate J0 was J0 ∼ 2 × 10 cm s. As the drying progressed further, the drop attained a constant base radius at t∼ 360 s, and a surface undulation appeared at the top of the drop. A thin crust appeared at the free surface, with a convective flow toward the foot underneath. The formation of the crust slowed down the solvent loss from the initial evaporation rate J0 (Fig. 1c) by reducing the diffusion of water from the core of the drop to the free surface. Loss of the solvent decreased the volume enclosed by the thin crust, and the crust thus inverted, forming an undulation similar to a collapsing dome. The drying process is summarized schematically in Figure 2. C O M M U N IC A IO N S

The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on porosity formation

Abstract: Modification of the eutectic Si in Al-Si foundry alloys by adding strontium or sodium is, unfortunately, accompanied by an increase of porosity in the casting. In an attempt to understand the nature of this problem, this study used a sessile-drop method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity occur-rence. The addition of 0.01 wt pct Sr and 0.005 wt pct Na to A356 alloy decreases the surface tension of the liquid by about 19 and 10 pct, respectively, and may increase the volume shrink-age by about 12 pct. These changes to surface tension and volumetric shrinkage promote the early formation of the pores during solidification and give the availability of a longer period of growth prior to complete solidification, resulting in a larger pore size. The effect of surface tension on the pores is more significant than volumetric shrinkage. Although the predicted pore diameter increases with lower surface tension or higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in porosity that is associated with modification.

Polar interactions at liquid/polymer interfaces

Abstract: Numerous relationships have been proposed in the literature to interpret wettability in terms of solid and liquid surface free energies. In the classical approach based on surface free energy components, the energy of interactions between the liquid and the solid is obtained from the geometric mean of the dispersion and polar contributions of the liquid and solid surface free energies. In this work, it is shown that the surface polarity of polar liquids can be modeled by the interaction of aligned permanent dipoles. A good agreement is found between the surface polarity characterized by polar component of the surface free energy of polar liquids (water, formamide and ethylene glycol) and the dipolar energy of interactions calculated from their dipole moment. At the liquid/polymer interfaces, polar interactions are better described by a simple relationship of proportionality with the polar component of the liquid surface free energy. This observation leads us to evaluate the hypothesis of induced polar int...

Wettability and interfacial interactions in bioceramic-body-liquid systems

Abstract: Wetting experiments, by the sessile drop technique, were carried out at 37°C in air to determine the surface and interfacial interactions that take place in various solid bioceramics based on Al2O3, ZrO2(YPZ), SiO2, and TiO2 in contact with water, Ringer solution, artificial synovial fluid, calf serum, human plasma, and whole blood (+ EDTA). The surface energy of the liquids was measured by the ring method. The calculated values of the energy of interaction (work of adhesion) reveal that intermolecular forces act across the solid-liquid interfaces. The contribution of the dispersion and polar interactions to the surface energy of the polar liquids and the pure or mixed oxides was determined assuming that in the system of Mn-steel-liquids only dispersion forces act at the interface. It was found that the contribution of the polar interactions to the energy of interaction at the solid-liquid interface increases with the glassy phase content of the oxide that causes reduction of the measured contact angle. © 1995 John Wiley & Sons, Inc.

The influence on surface free energy of PVD-coatings

Abstract: Surface free energy is a characteristic factor which affects the surface properties and interfacial interactions such as adsorption, wetting and adhesion. Surface free energy is thus of interest in the field of adhesive technologies, biomedical applications, cleaning procedures or for the wettability of tribological systems. One method of determining the polar and dispersive terms of the free surface energies of solids is based on measurements of the contact angles of pure liquids on solid surfaces. This paper describes the use of dynamic contact angle analysis for the evaluation of surface tension.