Showing papers on "Sessile drop technique published in 2008"

Comparison of different methods to measure contact angles of soil colloids

Abstract: We compared five different methods, static sessile drop, dynamic sessile drop, Wilhelmy plate, thin-layer wicking, and column wicking, to determine the contact angle of colloids typical for soils and sediments. The colloids (smectite, kaolinite, illite, goethite, hematite) were chosen to represent 1:1 and 2:1 layered aluminosilicate clays and sesquioxides, and were either obtained in pure form or synthesized in our laboratory. Colloids were deposited as thin films on glass slides, and then used for contact angle measurements using three different test liquids (water, formamide, diiodomethane). The colloidal films could be categorized into three types: (1) films without pores and with polar-liquid interactions (smectite), (2) films with pores and with polar-liquid interactions (kaolinite, illite, goethite), and (3) films without pores and no polar-liquid interactions (hematite). The static and dynamic sessile drop methods yielded the most consistent contact angles. For porous films, the contact angles decreased with time, and we consider the initial contact angle to be the most accurate. The differences in contact angles among the different methods were large and varied considerably: the most consistent contact angles were obtained for kaolinite with water, and illite with diiodomethane (contact angles were within 3 degrees); but mostly the differences ranged from 10 degrees to 40 degrees among the different methods. The thin-layer and column wicking methods were the least consistent methods.

Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: an experimental investigation.

Abstract: Sessile liquid drops are predicted to deform an elastic surface onto which they are placed because of the combined action of the liquid surface tension at the periphery of the drop and the capillary pressure inside the drop. Here, we show for the first time the in situ experimental confirmation of the effect of capillary pressure on this deformation. We demonstrate micrometer-scale deformations made possible by using a low Young's modulus material as an elastic surface. The experimental profiles of the deformed surfaces fit well the theoretical predictions for surfaces with a Young's modulus between 25 and 340 kPa.

Wettability Determination of the Reservoir Brine−Reservoir Rock System with Dissolution of CO2 at High Pressures and Elevated Temperatures

Abstract: An experimental method has been developed to determine the wettability, i.e., the contact angle, of a reservoir brine−reservoir rock system with dissolution of CO2 at high pressures and elevated temperatures by using the axisymmetric drop shape analysis (ADSA) technique for the sessile drop case. Prior to the experiment, a rock slide is horizontally placed in a specially designed rock slide holder in a see-through windowed high-pressure cell, which is subsequently filled with CO2 at a prespecified pressure and a constant temperature. Then, a reservoir brine sample is introduced by using a syringe delivery system to form a sessile brine drop on the rock slide inside the pressure cell. The sequential digital images of the dynamic sessile brine drop are acquired and analyzed by applying computer-aided image acquisition and processing techniques to measure the dynamic contact angles at different times. It is found that the dynamic contact angle between the reservoir brine and the reservoir rock remains almost...

Wetting and absorption of water drops on Nafion films.

Abstract: Water drops on Nafion films caused the surface to switch from being hydrophobic to being hydrophilic. Contact angle hysteresis of >70° between advancing and receding values were obtained by the Wilhelmy plate technique. Sessile drop measurements were consistent with the advancing contact angle; the sessile drop contact angle was 108°. Water drop adhesion, as measured by the detachment angle on an inclined plane, showed much stronger water adhesion on Nafion than Teflon. Sessile water and methanol drops caused dry Nafion films to deflect. The flexure went through a maximum with time. Flexure increased with contact area of the drop, but was insensitive to the film thickness. Methanol drops spread more on Nafion and caused larger film flexure than water. The results suggest that the Nafion surface was initially hydrophobic but water and methanol drops caused hydrophilic sulfonic acid domains to be drawn to the Nafion surface. Local swelling of the film beneath the water drop caused the film to buckle. The ma...

Shape Oscillation of a drop in ac electrowetting.

Abstract: A sessile drop oscillates when an ac voltage is applied in electrowetting. The oscillation results from the time-varying electrical force concentrated on the three-phase contact line. Little is known about the feature of drop oscillation in electrowetting. In the present work, the drop oscillations are observed systematically, and a theoretical model is developed to analyze the oscillation. It is revealed that resonance occurs at certain frequencies and the oscillation pattern is significantly dependent on the applied ac frequencies. The domain perturbation method is used to derive the shape-mode equations under the assumptions of a weak viscous effect and small drop deformation. The electrical force concentrated on the three-phase contact line is approximated as a delta function, which is decomposed and substituted into each shape-mode equation as a forcing term. The theoretical results for the shape and frequency responses are compared with experimental results, which shows qualitative agreement.

Characterizing water repellency indices: Contact angle and water drop penetration time of hydrophobized sand

Abstract: The accuracy and the reproducibility of water repellency measurements can be improved with the use of proper measurement techniques. The purpose of this study is to compare the contact angles obtained using three different methods and to examine the relationship between contact angle and water drop penetration time (WDPT) using non-repellent to extremely repellent sands. Fine silica sand, artificially hydrophobized with stearic acid, was used for the measurements. Contact angles were estimated using the molarity of an ethanol droplet (MED) test, the capillary rise method (CRM) and the sessile drop method (SDM). The CRM was effective for soils with contact angles < 90°. The MED test was suitable for soils with contact angles ≥ 90° using 10 s as the ethanol drop penetration time. The SDM was effective for soils with contact angles ranging from very low to very high (11–105°). Directly measured contact angles using the SDM were in good agreement with indirectly obtained contact angles using the MED ...

Particle and substrate charge effects on colloidal self-assembly in a sessile drop.

Abstract: By direct video monitoring of dynamic colloidal self-assembly during solvent evaporation in a sessile drop, we investigated the effect of surface charge on the ordering of colloidal spheres. The in situ observations revealed that the interaction between charged colloidal spheres and substrates affects the mobility of colloidal spheres during convective self-assembly, playing an important role in the colloidal crystal growth process. Both ordered and disordered growth was observed depending on different chemical conditions mediated by surface charge and surfactant additions to the sessile drop system. These different self-assembly behaviors were explained by the Coulombic and hydrophobic interactions between surface-charged colloidal spheres and substrates.

Experimental complex for investigations of high temperature capillarity phenomena

Abstract: The paper presents the description of an experimental complex that has been designed for investigations of high temperature capillarity phenomena by various testing methods (classical sessile drop, pendant drop, dispensed drop, sandwiched drop, transferred drop, drop sucking, drop pushing, drop smearing or rubbing) at a temperature of up to 2100 °C under vacuum of up to 10−7 hPa or in protective atmosphere (static or flowing gas with controlled rate at required level of pressure). Several examples of high temperature wettability tests are discussed in order to demonstrate the wide testing possibilities of the new experimental complex.

Particle Deposition Study During Sessile Drop Evaporation

Abstract: The focus of this article is the numerical study of particle deposition profiles on a solid substrate during the evaporation of a sessile drop of a colloidal particle suspension. The evaporation flux along the drop interface, the induced fluid dynamics inside the drop, and the particle deposition profile on the solid substrate are solved simultaneously. The governing equations are solved numerically using the Galerkin/finite element method (G/FEM) for discretization of the spatial domain and an adaptive finite difference method for discretization in the time domain. Several particle deposition profiles, such as a ring-shaped deposit and uniform particle distribution, are obtained from the numerical simulations. The particle deposition profile is found to be influenced by the mass transfer (both convective and diffusive mass transfer) of the particles in the bulk liquid and by the deposition rate along the substrate. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Wettability of carbon surfaces by pure molten alkali chlorides and their penetration into a porous graphite substrate

Abstract: The wettability of graphite and glassy carbon surfaces by pure molten alkali chlorides (NaCl, KCl, RbCl, CsCl) was measured by the sessile drop method. The contact angle was found to decrease with increase of the cation radius of the chloride. Using our measured and available literature data, a new, semi-empirical model is established to estimate the adhesion energy between the 20 alkali halide molten salts and graphite (or glassy carbon). The adhesion energy is found to increase with square of the radius of the cation, and the inverse of the radius of the anion of the salt. The minimum possible value for the surface energy of graphite (and glassy carbon) was found as 150 ± 30 mJ/m 2 . The critical contact angle of spontaneous penetration (infiltration) of the molten chlorides into a porous graphite substrate was found experimentally below 90 ◦ , in the interval between 31 ◦ and 58 ◦ . This is explained by the inner structure of the porous graphite.

Wetting and brazing of stainless steels by copper–silver eutectic

Abstract: The wetting of stainless steels by Cu–Ag-based alloys was studied by the sessile drop and dispensed drop techniques in high vacuum at 800–900 °C. Experiments were performed by varying the steel type, composition of the Cu–Ag-based alloy and furnace atmosphere. The results were used to determine the processes controlling the wetting and brazing of these important technological materials.

Wetting in Soldering and Microelectronics

Abstract: Wettability of solid metals by molten solders is reviewed. The contact angle and wetting force are tabulated for various combinations of solid metals and molten solders such as Sn-Pb base alloys, Sn-Ag base alloys, Sn-Zn base alloys, Sn-Cu base alloys, and Sn-Bi base alloys. Studies on the wetting rate are also discussed.

Thermosensitive Copolymer Coatings with Enhanced Wettability Switching

Abstract: Expanded cross-linked copolymers of poly(N-isopropylacrylamide) (PNiPAAm) and poly(acrylic acid) (PAAc) of varying monomer ratios were grafted from a crystalline silicon surface. Surface-tethered polymerization was performed at a slightly basic pH, where electrostatic repulsion among acrylic acid monomer units forces the network into an expanded polymer conformation. The influence of this expanded conformation on switchability between a hydrophilic and a hydrophobic state was investigated. Characterization of the copolymer coating was carried out by means of X-ray photoelectron spectroscopy (XPS) ellipsometry, and diffuse reflectance IR. Lower critical solution temperatures (LCSTs) of the copolymer grafts on the silicon surfaces were determined by spectrophotometry. Temperature-induced wettability changes were studied using sessile drop contact angle measurements. The surface topography was investigated by atomic force microscopy (AFM) in Milli-Q water at 25 and 40 °C. The reversible attachment of a fluorescently labeled model protein was studied as a function of temperature using a fluorescence microscope and a fluorescence spectrometer. Maximum switching in terms of the contact angle change around the LCST was observed at a ratio of 36:1 PNiPAAm to PAAc. The enhanced control of biointerfaces achieved by these coatings may find applications in biomaterials, biochips, drug delivery, and microfluidics.

PEGylation of porous silicon using click chemistry.

Abstract: Porous silicon has received considerable interest in recent years in a range of biomedical applications, with its performance determined by surface chemistry. In this work, we investigate the PEGylation of porous silicon wafers using click chemistry. The porous silicon wafer surface chemistry was monitored at each stage of the reaction via photoacoustic Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, whereas sessile drop contact angle and model protein adsorption measurements were used to characterize the final PEGylated surface. This work highlights the simplicity of click-chemistry-based functionalization in tailoring the porous silicon surface chemistry and controlling protein−porous silicon interactions.

Analytical solution of Stokes flow inside an evaporating sessile drop: Spherical and cylindrical cap shapes

Abstract: Exact analytical solutions are derived for the Stokes flows within evaporating sessile drops of spherical and cylindrical cap shapes. The results are valid for arbitrary contact angle. Solutions are obtained for arbitrary evaporative flux distributions along the free surface as long as the flux is bounded at the contact line. The field equations, E^4(Psi)=0 and Del^4(Phi)=0, are solved for the spherical and cylindrical cap cases, respectively. Specific results and computations are presented for evaporation corresponding to uniform flux and to purely diffusive gas phase transport into an infinite ambient. Wetting and non-wetting contact angles are considered with the flow patterns in each case being illustrated. For the spherical cap with evaporation controlled by vapor phase diffusion, when the contact angle lies in the range 0

The surface tension of liquid silicon at high temperature

Abstract: The measurement of the surface tension of liquid silicon has a long history with many results but no general agreement between them. Two values at the melting temperature are cited in reviews (749 and 827 mN/m [N. Eustathopoulos, E. Ricci, B. Drevet, Note Technique DEM No. 97/58, CEA, 1997]) but there are few arguments to determine the correct one. In the present study, new data for the surface tension obtained with the analysis of characteristic frequencies of a levitating drop are presented. The effect of oxygen and nitrogen are also considered. These data are compared with former data obtained with contactless techniques. The most recent surface tension values obtained with drop weights ranging on two orders of magnitude and environments of different natures (argon, hydrogen and vacuum) show excellent agreement (within a 1.5% margin) at temperatures between 1350 K and 2400 K. The comparison of these data to others obtained with different techniques, reveal a good agreement, except those obtained with the sessile drop technique on some supports like BN, SiO2 and MgO. However, these special cases may be connected with the reactivity of silicon with these supports.

Wetting of pure aluminum and selected alloys on polycrystalline alumina and sapphire

Abstract: Using a modification of the dispensed drop method to measure true contact angles of readily oxidizing metals and alloys, the wettability of polycrystalline alumina and A-plane sapphire by pure aluminum and selected aluminum alloys was investigated. The experiments were performed under high vacuum in a horizontal tube furnace. The experimental setup produces a sessile drop free of its natural surface oxide layer minimizing flight time of the drop, and maintaining a drop impingement on the substrate. The experiments showed that there is no significant difference in the wettability of alumina and sapphire by aluminum as well as Al–11.5Si, Al–1Mg and Al–7Cu. On both substrates, aluminum shows a strong increase in contact angle well into the non-wetting regime just above the melting point. The wetting behavior of Al–7Cu on both substrates is slightly but significantly reduced in comparison to pure aluminum. The contact angles of Al–1Mg and Al–11.5Si remain rather constant between the respective liquidus temperatures of the alloys and 800 °C with θ (Al–1Mg) θ (Al–11.5Si). Only Al–7Cu above 730 °C achieves the contact angle interval of 70–86° suggested to be most beneficial in terms of aluminum foam stabilization.

Characterization of Dynamic Stick-and-Break Wetting Behavior for Various Liquids on the Surface of a Highly Viscoelastic Polymer

Abstract: A thermally stripped acrylic polymer was wet with a series of liquids possessing a broad range of properties. Previously, novel wetting behavior by water was reported for the polymer, which included the formation of a wetting ridge structure substantially larger than those reported elsewhere and the complete halting of the three-phase line. This allows metastable angles ranging from 0° to greater than 150° to be achieved through changes in the sessile drop volume. Greater advancing angles are prevented by the collapse of the drop, producing what has been described as stick-and-break propagation. In Wilhelmy plate experiments for metal plates coated with the polymer, this mechanism produces a quasi-periodic pattern of lines composed of ridge structures. Similar behavior was observed for all liquids tested. Differences were observed in the maximum force measured with a tensiometer (pinning force) and the average distance between ridges for the formed pattern (pinning distance). These quantities are shown to...

Reactive wetting in metal/metal systems : Dissolutive versus compound-forming systems

Abstract: Reactive wetting in metal–metal systems involves complex interactions between the molten phase and the solid substrate. The simplest interaction involves only the dissolution of the substrate into the molten phase. The more complex interaction involves both dissolution of the substrate and compound formation at the solid–liquid boundary. The fundamental differences between these two types of reactive wetting are identified by studying the purely dissolutive system Bi–Sn and the compound-forming system Au–Sn. Experiments employ the traditional sessile drop technique as well as a novel two-dimensional drop transfer technique that enables real-time visualization of the solid–liquid interface evolution. Recent results from wetting of pure Sn on Au-coated Cu substrates are also presented and reveal a much richer wetting behavior than either Sn on Au or Sn on Cu binary systems.

Dissolutive wetting of Si by molten Cu

Abstract: Dissolutive wetting occurs when a liquid spreads over a solid surface with simultaneous dissolution of the solid into the liquid. This process is of great interest for both fundamental research and several industrial processes, an important example being soldering in microelectronics fabrication processes [1]. Several studies, performed for various liquid metal/solid metal systems, have shown that for millimetresized droplets the spreading time in dissolutive wetting ranges from a few to several hundred seconds [2–6]. This time is orders of magnitude higher than the spreading time found in liquid metal/solid metal systems with negligible miscibility, which is typically around 10 ms [7–11]. Despite the progress made over the last 10 years in the understanding of dissolutive wetting, several points remain obscure concerning both the driving force and kinetics of this type of wetting. The aim of the work reported in this paper is to contribute to this subject by performing a comparative study of spreading on the same substrate (monocrystalline Si) at 1100 C with pure copper and Si-presaturated copper. In the past, a similar attempt to study spreading in the same system in equilibrium and non-equilibrium conditions was made using Ag/Cu couple [12]. However, with the sessile drop technique used in these experiments, the initial stages of spreading were obscured by metal melting. In the present study this difficulty is overcome by using the dispensed drop technique, which enables the processes of melting and spreading to be separated (see for instance [13]) According to the Cu–Si phase diagram, at 1100 C a liquid CuSi alloy containing 52 at %Si is in equilibrium with solid Si (the solubility of Cu into solid Si is negligible) [14]. As for the surface tension of pure Cu at 1100 C, 1280 mN/m [15], it is much higher than that of molten Si, which, extrapolated to 1100 C from the melting point of Si, is close to 800 mN/m [16, 17]. As a consequence, dissolution of Si into Cu is expected to decrease the surface tension of the liquid. Wetting was studied in a metal furnace under a vacuum of (1–5) 9 10 Pa. The experiment involved heating pure Cu or the CuSi alloy (purity higher than 99.999%) in an alumina crucible placed above the Si substrate. At the experimental temperature, the liquid was extruded from the crucible through a capillary, forming droplets with a diameter ddr lying between 1.3 and 2 mm. In view of the high sensitivity of Si to oxidation and to improve surface cleaning, a prior heat treatment of the substrates was performed at 1250 C before depositing the drop at 1100 C. The wetting process was filmed by a camera (500 frames per second) connected to a computer, enabling automatic image analysis. The characteristic dimensions of the drop (drop base diameter d and visible contact angle h) were extracted with an accuracy of ±2 for h and ±2% for d. The (111) surfaces of electronic purity Si have an average roughness of 1–4 nm after polishing with diamond paste up to 0.1 lm. Figure 1 gives the temporal variation in contact angle h and the normalized drop base diameter d/ddr for the wetting of Si by a Cu droplet. Due to the resolution of 2 ms, most of the non-reactive spreading, which occurs at t \ 2 ms, is missing. However, on the drop base diameter curve, it can be clearly seen that the triple line velocity vanishes at point A, corresponding to time t & 4 ms. This is just a tendency because, after slightly receding to point B, d starts to P. Protsenko Department of Colloid Chemistry, MSU, Moscow, Russia

Microstructuring of polystyrene surfaces with nonsolvent sessile droplets

Abstract: Herein, we study the microstructuring of toluene-vapor-softened polystyrene surfaces with nonsolvent sessile droplets. Arrays of microvessels are obtained by depositing non-evaporating droplets of ethylene glycol/water on the original polystyrene surfaces and subsequently exposing them to saturated toluene vapor. The droplets act as a mask on the polymer, thereby impeding the toluene vapor to diffuse and soften the polystyrene surface below them. Alternatively, the formation of microcraters at random positions-with an average depth-to-width aspect ratio of 0.5 and a diameter as small as 1.5 mu m-is achieved by condensing water droplets on a softened polystyrene surface. The cross-sections of the microvessels and the contact angle of the sessile water droplets suggest that the structures are formed by the combined action of the Laplace pressure at the bottom of the droplet and the surface tension acting at the three-phase contact line of the droplets. As a support, the rim height and the depth of the microvessels are fitted with an elastic theory to provide Young's modulus of the softened polystyrene surface.

Relative humidity effects on contact angle and water drop penetration time of hydrophobized fine sand

Abstract: The purpose of this study was to determine the effects of ambient relative humidity (RH) on contact angle and water drop penetration time (WDPT) using a series of sand samples hydrophobized with stearic acid. The contact angle was estimated using the sessile drop method. The contact angle increased first sharply and then slightly with increasing stearic acid content up to about 5.0 g kg−1. The contact angle did not change with increasing stearic acid content above 5.0 g kg−1. The contact angle increased with increasing RH from 33 to 94%. The RH did not affect the contact angle of samples with a stearic acid content above 5.0 g kg−1, where the particles were considered to be completely coated by hydrophobic organic material. The contact angle increased with increasing RH in sands partially coated with hydrophobic material, which might have resulted from an increase in adsorbed water molecules on the remaining high-energy mineral surfaces. The WDPT increased with increasing RH. The higher the RH, t...