Showing papers on "Sessile drop technique published in 2009"

Ultrafast microfluidics using surface acoustic waves

Abstract: We demonstrate that surface acoustic waves (SAWs), nanometer amplitude Rayleigh waves driven at megahertz order frequencies propagating on the surface of a piezoelectric substrate, offer a powerful method for driving a host of extremely fast microfluidic actuation and microbioparticle manipulation schemes. We show that sessile drops can be translated rapidly on planar substrates or fluid can be pumped through microchannels at 1-10 cms velocities, which are typically one to two orders quicker than that afforded by current microfluidic technologies. Through symmetry-breaking, azimuthal recirculation can be induced within the drop to drive strong inertial microcentrifugation for micromixing and particle concentration or separation. Similar micromixing strategies can be induced in the same microchannel in which fluid is pumped with the SAW by merely changing the SAW frequency to rapidly switch the uniform through-flow into a chaotic oscillatory flow by exploiting superpositioning of the irradiated sound waves from the sidewalls of the microchannel. If the flow is sufficiently quiescent, the nodes of the transverse standing wave that arises across the microchannel also allow for particle aggregation, and hence, sorting on nodal lines. In addition, the SAW also facilitates other microfluidic capabilities. For example, capillary waves excited at the free surface of a sessile drop by the SAW underneath it can be exploited for micronanoparticle collection and sorting at nodal points or lines at low powers. At higher powers, the large accelerations off the substrate surface as the SAW propagates across drives rapid destabilization of the drop free surface giving rise to inertial liquid jets that persist over 1-2 cm in length or atomization of the entire drop to produce 1-10 mum monodispersed aerosol droplets, which can be exploited for ink-jet printing, mass spectrometry interfacing, or pulmonary drug delivery. The atomization of polymerprotein solutions can also be used for the rapid synthesis of 150-200 nm polymerprotein particles or biodegradable polymeric shells in which proteins, peptides, and other therapeutic molecules are encapsulated within for controlled release drug delivery. The atomization of thin films behind a translating drop containing polymer solutions also gives rise to long-range spatial ordering of regular polymer spots whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for polymer patterning without requiring surface treatment or physicalchemical templating.

On the effect of the atmosphere on the evaporation of sessile droplets of water

Abstract: An experimental and theoretical study of the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described. The experimental work investigated the evaporation rates of sessile droplets in atmospheres of three different ambient gases (namely, helium, nitrogen, and carbon dioxide) at reduced pressure (from 40 to 1000 mbars) using four different substrates (namely, aluminum, titanium, Macor, and polytetrafluoroethylene) with a wide range of thermal conductivities. Reducing the atmospheric pressure increases the diffusion coefficient of water vapor in the atmosphere and hence increases the evaporation rate. Changing the ambient gas also alters the diffusion coefficient and hence also affects the evaporation rate. A mathematical model that takes into account the effect of the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapor in the atmosphere and the thermal conductivity of the substrate is developed, and its predictions are found to be in encouraging agreement with the experimental results.

Surface forces and wetting phenomena

Abstract: Conditions for thermodynamic equilibrium of liquid drops on solid substrates are presented. It is shown that if surface force (disjoining/conjoining Derjaguin pressure) action in a vicinity of the three-phase contact line is taken into account the condition of thermodynamic equilibrium is duly satisfied. Then the thermodynamic expressions for equilibrium contact angles of drops on solid substrates and menisci in thin capillaries are expressed in terms of the corresponding Derjaguin isotherm. It is shown that equilibrium contact angles of drops vary significantly depending on the vapour pressure in the ambient atmosphere, while there is a single, unique equilibrium contact angle in thin capillaries. It is also shown that the static advancing contact angle of a drop depends on its volume, in agreement with experimental data. In the case of menisci in capillaries, the expression for the receding contact angle is deduced, with results that are also in agreement with known experimental data.

Evaporation and fluid dynamics of a sessile drop of capillary size.

Abstract: Theoretical description and numerical simulation of an evaporating sessile drop are developed. We jointly take into account the hydrodynamics of an evaporating sessile drop, effects of the thermal conduction in the drop, and the diffusion of vapor in air. A shape of the rotationally symmetric drop is determined within the quasistationary approximation. Nonstationary effects in the diffusion of the vapor are also taken into account. Simulation results agree well with the data of evaporation rate measurements for the toluene drop. Marangoni forces associated with the temperature dependence of the surface tension generate fluid convection in the sessile drop. Our results demonstrate several dynamical stages of the convection characterized by different number of vortices in the drop. During the early stage the array of vortices arises near a surface of the drop and induces a nonmonotonic spatial distribution of the temperature over the drop surface. The initial number of near-surface vortices in the drop is controlled by the Marangoni cell size which is similar to that given by Pearson for flat fluid layers. This number quickly decreases with time resulting in three bulk vortices in the intermediate stage. The vortices finally transform into the single convection vortex in the drop existing during about 1/2 of the evaporation time.

Vibrations of sessile drops

Abstract: This study focuses on the effects of vertical vibrations on sessile drops deposited on hydrophobic substrates. At low amplitudes the contact line remains pinned because of contact angle hysteresis and only drop surface modes areobserved. Above a first threshold the contact line starts to move and exhibits a stick-slip behavior that presents some analogies with the solid friction on amechanical oscillator. At larger amplitudes, non-axisymmetric contour modes show up (modes m=2, 3...). They can be interpreted as a coupling between surface modes and contact line motion. These subharmonic modes are welldescribed within the framework of parametric oscillators. We also discuss here why vibrations can help to measure equilibrium contact angle.

Wetting and adhesion of Si on Si3N4 and BN substrates

Abstract: Wetting, adhesion and reactivity are the principal factors determining the capability of a solid to be used as mould material. In this work wetting of silicon and boron nitrides by molten silicon is studied in neutral gas atmosphere by the sessile drop technique at temperatures close to the silicon melting point. Adhesion is qualified by the behaviour of solidified droplets under the effect of thermo-mechanical stresses generated during cooling at room temperature. The reactivity at silicon/nitride interfaces is studied by scanning electron microscopy and EDX-microanalysis.

Sessile Drop in Microgravity: Creation, Contact Angle and Interface

Abstract: We present in this paper the results obtained from a parabolic flight campaign regarding the contact angle and the drop interface behavior of sessile drops created under terrestrial gravity (1g) or in microgravity (μg). This is a preliminary study before further investi- gations on sessile drops evaporation under micrograv- ity. In this study, drops are created by the mean of a syringe pump by injection through the substrate. The created drops are recorded using a video camera to extract the drops contact angles. Three fluids have been used in this study : de-ionized water, HFE-7100 and FC- 72 and two heating surfaces: aluminum and PTFE. The results obtained evidence the feasibility of sessile drop creation in microgravity even for low surface tension liquids (below 15 mN m −1 )s uch as FC-72 and HFE- 7100. We also evidence the contact angle behavior depending of the drop diameter and the gravity level. A second objective of this study is to analyze the drop interface shape in microgravity. The goal of the these experiments is to obtain reference data on the sessile drop behavior in microgravity for future experiments to be performed in an French-Chinese scientific instru- ment (IMPACHT).

Effect of sessile drop volume on the wetting anisotropy observed on grooved surfaces.

Abstract: This study reports experimental measurements of the water contact angle (WCA) measured on surfaces with grooves of different widths using drop volumes ranging from 400 pL to 4.5 μL. These measurements were carried out on both relatively hydrophobic and hydrophilic surface chemistry formed using a conformal plasma polymer coating of topographically embossed poly(methyl methacrylate) (PMMA). Anisotropic wetting of the grooved surfaces was found to be more marked for larger drops on both the hydrophilic and hydrophobic surfaces. Above a certain drop base diameter to groove width ratio, topography had no effect on the measured WCA; this ratio was found to be dependent on the water drop volume. The WCA measured from the direction perpendicular to the grooves using submicroliter water drops is found to be a good indicator of the WCA on the flat surface with equivalent wettabilities. To the best of our knowledge, this is the first study on the phenomenon of anisotropic wetting using picoliter water drops.

Wetting and evaporation behaviors of water-ethanol sessile drops on PTFE surfaces

Abstract: The wetting and evaporation behaviors of water–ethanol mixtures on polished poly(tetrafluoroethylene) (PTFE) surfaces were studied with an emphasis on the influence of concentration. Three representative stages: (i) constant contact diameter with decreasing contact angle and drop height; (ii) increase in contact angle and drop height with decreasing contact diameter; and (iii) simultaneous decrease in contact angle, drop height, and contact diameter, were identified in the wetting process under evaporation. In the initial stage, with increasing ethanol concentration, the wetting and evaporating behaviors of the mixture drops are close to those of pure ethanol, while in the final stage they are close to those of pure water, regardless of the concentration. The most significant difference between the pure substance and mixture drops is in the intermediate stage due to the different controlling mechanisms behind the de-pinning phenomenon. Moreover, the concrete wetting behavior is dependent on the composition of the mixture drops. For an evaporating sessile drop, the wetting should be evaluated using the combined parameters of contact angle, drop height, and contact diameter. Six representative modes are proposed to describe the wetting behaviors of the water–ethanol sessile drops on the PTFE surfaces under evaporation. Copyright © 2009 John Wiley & Sons, Ltd.

Superhydrophobic surfaces: a model approach to predict contact angle and surface energy of soil particles

Abstract: Wettability of soil affects a wide variety of processes including infiltration, preferential flow and surface runoff. The problem of determining contact angles and surface energy of powders, such as soil particles, remains unsolved. So far, several theories and approaches have been proposed, but formulation of surface and interfacial free energy, as regards its components, is still a very debatable issue. In the present study, the general problem of the interpretation of contact angles and surface free energy on chemically heterogeneous and rough soil particle surfaces is evaluated by a reformulation of the Cassie-Baxter equation, assuming that the particles are attached on to a plane and rigid surface. Compared with common approaches, our model considers a roughness factor that depends on the Young’s Law contact angle determined by the surface chemistry. Results of the model are discussed and compared with independent contact angle measurements using the Sessile Drop and the Wilhelmy Plate methods. Based on contact angle data, the critical surface tension of the grains were determined by the method proposed by Zisman. Experiments were made with glass beads and three soil materials ranging from sand to clay. Soil particles were coated with different loadings of dichlorodimethylsilane (DCDMS) to vary the wettability. Varying the solid surface tension using DCDMS treatments provided pure water-wetting behaviours ranging from wettable to extremely hydrophobic, with contact angles > 150°. Results showed that the critical surface energy measured on grains with the highest DCDMS loadings was similar to the surface energy measured independently on ideal DCDMS-coated smooth glass plates, except for the clay soil. Contact angles measured on plane surfaces were related to contact angles measured on rough grain surfaces using the new model based on the combined Cassie-Baxter Wenzel equation, which takes into account the particle packing density on the sample surface.

Analytical solution for 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 all contact angles Solutions are obtained for arbitrary evaporative flux distributions along the free surface as long as the flux is bounded at the contact line 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 nonwetting 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≤θc<π/2, the mass flux of vapor becomes singular at the contact line This condition requires modification when solving for the liquid-phase transport Droplets in all of the above categories are considered for the following two cases: the contact lines are either pinned or free to move during evaporation

Stick-slip dynamics of an oscillated sessile drop

Abstract: We consider theoretically the dynamics of an oscillated sessile drop of incompressible liquid and focus on the contact line hysteresis. We address the situation of the small-amplitude and high-frequency oscillations imposed normally to the substrate surface. We deal with the drop whose equilibrium surface is hemispherical and the equilibrium contact angle equals π/2. We apply the dynamic boundary condition that involves an ambiguous dependence of the contact angle on the contact line velocity: The contact line starts to slide only when the deviation of the contact angle exceeds a certain critical value. As a result, the stick-slip dynamics can be observed. The frequency response of surface oscillations on the substrate and at the pole of the drop are analyzed. It is shown that novel features such as the emergence of antiresonant frequency bands and nontrivial competition of different resonances are caused by contact line hysteresis.

Stick-slip dynamics of an oscillated sessile drop

Abstract: The dynamics of an oscillated sessile drop of incompressible liquid with the focus on the contact line hysteresis is under theoretical consideration The solid substrate is subject to transverse oscillations, which are assumed small amplitude and high frequency The dynamic boundary condition that involves an ambiguous dependence of the contact angle on the contact line velocity is applied: the contact line starts to move only when the deviation of the contact angle exceeds a certain critical value As a result, the stick-slip dynamics can be observed The frequency response of surface oscillations on the substrate and at the pole of the drop are analyzed It is shown that novel features such as the emergence of antiresonant frequency bands and nontrivial competition of different resonances are caused by contact line hysteresis

Measurement errors related to contact angle analysis of hydrogel and silicone hydrogel contact lenses.

Abstract: This work sought to undertake a comprehensive investigation of the measurement errors associated with contact angle assessment of curved hydrogel contact lens surfaces. The contact angle coefficient of repeatability (COR) associated with three measurement conditions (image analysis COR, intralens COR, and interlens COR) was determined by measuring the contact angles (using both sessile drop and captive bubble methods) for three silicone hydrogel lenses (senofilcon A, balafilcon A, lotrafilcon A) and one conventional hydrogel lens (etafilcon A). Image analysis COR values were about 2 degrees , whereas intralens COR values (95% confidence intervals) ranged from 4.0 degrees (3.3 degrees , 4.7 degrees ) (lotrafilcon A, captive bubble) to 10.2 degrees (8.4 degrees , 12.1 degrees ) (senofilcon A, sessile drop). Interlens COR values ranged from 4.5 degrees (3.7 degrees , 5.2 degrees ) (lotrafilcon A, captive bubble) to 16.5 degrees (13.6 degrees , 19.4 degrees ) (senofilcon A, sessile drop). Measurement error associated with image analysis was shown to be small as an absolute measure, although proportionally more significant for lenses with low contact angle. Sessile drop contact angles were typically less repeatable than captive bubble contact angles. For sessile drop measures, repeatability was poorer with the silicone hydrogel lenses when compared with the conventional hydrogel lens; this phenomenon was not observed for the captive bubble method, suggesting that methodological factors related to the sessile drop technique (such as surface dehydration and blotting) may play a role in the increased variability of contact angle measurements observed with silicone hydrogel contact lenses.

Analytical solution for inviscid flow inside an evaporating sessile drop.

Abstract: Inviscid flow within an evaporating sessile drop is analyzed. The field equation E;{2}psi=0 is solved for the stream function. The exact analytical solution is obtained for arbitrary contact angle and distribution of evaporative flux along the free boundary. Specific results and computations are presented for evaporation corresponding to both uniform flux and purely diffusive gas phase transport into an infinite ambient. Wetting and nonwetting contact angles are considered, with flow patterns in each case being illustrated. The limiting behaviors of small contact angle and droplets of hemispherical shape are treated. All of the above categories are considered for the cases of droplets whose contact lines are either pinned or free to move during evaporation.

Time dependence of contact angle and its relation to repellency persistence in hydrophobized sand.

Abstract: Soil water repellency is a transient soil property varying with soil–water contact time. The purpose of the present study was to determine the time dependence of the sessile drop contact angle and its relation to repellency persistence estimated using the water drop penetration time (WDPT) test with hydrophobized sand. The contact angle decreased exponentially and almost reached apparent equilibrium after 20 min of soil–water contact time. Time dependence of the contact angle can mainly be attributed to the adsorption of water molecules onto low-energy hydrophobic organic matter surfaces. Contact angles initially greater than 90° decreased to less than 90° within about 40 s. However, the WDPT of these samples was longer than 3600 s. The WDPT responded to the initial contact angle, but not to the contact angle decreased with soil–water contact time. This was considered to be caused by differences in the surface free energy between the surface and the lower layers. Repellency persistence, or the WD...

Wetting and interfacial behavior of Ni–Si alloy on different substrates

Abstract: Wetting of molten Ni–56 at.% Si alloy on different substrates (SiC ceramic, Ni- and Co-based superalloys, Kovar, and Mo) are performed under different experimental conditions by the sessile drop technique. Temperature, atmosphere, and substrate composition play the key roles in determining the wettability, the spreading characteristics, and the interfacial morphology of the final interfaces. The non-reactive wetting characteristics in Ni–Si/SiC system are confirmed, with a spreading rate increasing with temperature increasing. In the Ni–Si/metal systems the spreading process is determined by the competition between spreading along the substrate surface and the interfacial interactions. Excellent wettability and fast spreading are found in the Ni–Si/Co-based superalloy, Ni–Si/Kovar, and Ni–Si/Mo systems at both the temperatures (1100 and 1200 °C). These results can be used as a reference guide for joining SiC to these metallic components, or to itself, using the Ni–Si alloy as filler metal.

Degree of Undercooling and Contact Angle of Pure Iron at 1933 K on Single-crystal Al2O3, MgO, and MgAl2O4 under Argon Atmosphere with Controlled Oxygen Partial Pressure

Abstract: Solid or molten oxides are considered to act as nucleation sites during continuous steel casting. The influence of the kind of oxide crystal and atmospheric oxygen partial pressure on the degree of undercooling and contact angle of pure molten iron on oxide substrates was measured by the sessile drop method. On Al2O3 and MgAl2O4 substrates, we found that the change in the degree of undercooling was dependent on the existence of a reaction layer and its thickness. On an MgO substrate, since no reaction layer formed, the degree of undercooling was small and was governed by the lattice misfit parameter. The equilibrium contact angle of a molten iron drop on the FeAl2O4 layer formed between the Al2O3 substrate and drop was about 100°; a similar contact angle was obtained on MgAl2O4. The contact angle on MgO changed during observation due to the evaporation of Mg.

An investigation of microbial adhesion to natural and synthetic polysaccharide-based films and its relationship with the surface energy components.

Abstract: In recent years, polysaccharide-based films have been developed for many applications. Some of these are in the pharmaceutical industry, where the adhesion of microorganisms to surfaces is a concern. After adhesion of a microorganism to a solid surface has taken place, the subsequent biofilm formed can act as a vehicle for spreading infections. The aim of this study is to compare the bacterial adhesion of E. coli and S. aureus from a contaminated solid model (Tryptone Soya Agar) to a range of polysaccharide-based films. These polysaccharide-based films consist of different natural starches (potato, cassava, wheat, pea and rice) and synthetic polymers hydroxyl-propyl cellulose (HPC) and carboxyl methyl cellulose (CMC)). The surface energy parameters of the films were calculated from the contact angle measurements by the sessile drop method. Apolar and polar liquids (water, formamide and hexadecane) and the Lifshitz-Van der Waals/acid-base (LW/AB) approach were used according to the method of Van Oss, Chaundhury and Good. The surface properties of the films were also correlated to the microbial adhesion. This indicated that, for both E. coli and S. aureus, the surface roughness did not affect the microbial adhesion. Only g\textS\textAB SABhad any correlation with the microbial adhesion and g\textS\textLW SLWwas almost constant for all the various polysaccharide films tested. In addition, the electron—donor properties of the materials, exhibited via g\textS + +S, were positively correlated with the adhesion of S. aureus but not with E. coli. This was in agreement with the results of the MATS (Microbial Adhesion To Solvents) test performed on the two bacteria. This revealed that only S. aureus presented an electron—acceptor characteristic.