Showing papers on "Sessile drop technique published in 2007"

How Wenzel and Cassie were wrong

Abstract: We argue using experimental data that contact lines and not contact areas are important in determining wettability. Three types of two-component surfaces were prepared that contain "spots" in a surrounding field: a hydrophilic spot in a hydrophobic field, a rough spot in a smooth field, and a smooth spot in a rough field. Water contact angles were measured within the spots and with the spot confined to within the contact line of the sessile drop. Spot diameter and contact line diameter were varied. All of the data indicate that contact angle behavior (advancing, receding, and hysteresis) is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the contact perimeter is irrelevant. The point is made that Wenzel's and Cassie's equations are valid only to the extent that the structure of the contact area reflects the ground state energies of contact lines and the transition states between them.

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...

In vitro water wettability of silicone hydrogel contact lenses determined using the sessile drop and captive bubble techniques.

Abstract: This study investigated the water contact angles of five commercially available silicone hydrogel contact lenses (Acuvue Advance, Acuvue Oasys, Focus Night & Day, O2 Optix, and PureVision) using sessile drop and captive bubble techniques. The only lens type that showed a significant difference in water contact angle when measured by sessile drop direct from the blister compared with after 48 h of soaking/washing in saline was the Acuvue Advance lens (from 66 degrees to 96 degrees, respectively) (p=0.0002), presumably because of surface active agents within the blister solution. The water contact angle data split the lenses into two distinct groups (p

Picoliter water contact angle measurement on polymers.

Abstract: Water contact angle measurement is the most common method for determining a material's wettability, and the sessile drop approach is the most frequently used. However, the method is generally limited to macroscopic measurements because the base diameter of the droplet is usually greater than 1 mm. Here we report for the first time on a dosing system to dispense smaller individual droplets with control of the position and investigate whether water contact angles determined from picoliter volume water droplets are comparable with those obtained from the conventional microliter volume water droplets. This investigation was conducted on a group of commonly used polymers. To demonstrate the higher spatial resolution of wettability that can be achieved using picoliter volume water droplets, the wettability of a radial plasma polymer gradient was mapped using a 250 μm interval grid.

The effect of polymer surface on the wetting and adhesion of liquid systems

Abstract: Young's equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid–solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined dep...

The initial coalescence of miscible drops

Abstract: When two drops of different miscible liquids are brought into contact, their coalescence speed is governed by the liquid having the weaker surface tension. Marangoni waves propagate along the drop with the stronger surface tension. We present surface profiles and propagation speeds of these waves, from experiments with a pendent water drop coalescing with a flat ethanol surface or with a sessile drop of ethanol. We find in the former case that the capillary-Marangoni waves along the water drop show self-similar character when measured in terms of arc length along the original surface. The coalescence of two liquids of different viscosities is also studied. For large viscosity difference, mobility is confined to the lower viscosity fluid and a sharp corner forms where the two liquids meet along the free surface. The coalescence speed of a water drop with a much more viscous liquid is nearly independent of the strength of the viscosity difference.

Asymmetric electrowetting—moving droplets by a square wave

Abstract: Here droplet oscillation and continuous pumping are demonstrated by asymmetric electrowetting on an open surface with embedded electrodes powered by a square wave electrical signal without control circuits. The polarity effect of electrowetting on an SU-8 and Teflon coated electrode is investigated, and it is found that the θ–V (contact angle–applied voltage) curve is asymmetric along the V = 0 axis by sessile drop and coplanar electrode experiments. A systematic deviation of measured contact angles from the theoretical ones is observed when the electrode beneath the droplet is negatively biased. In the sessile drop experiment, up to a 10° increment of contact angle is measured on a negatively biased electrode. In addition, a coplanar electrode experiment is designed to examine the contact angles at the same applied potential but opposite polarities on two sides of one droplet at the same time. The design of the coplanar electrodes is then expanded to oscillate and transport droplets on square-wave-powered symmetric (square) and asymmetric (polygon) electrodes to demonstrate manipulation capability on an open surface. The frequency of oscillation and the speed of transportation are determined by the frequency of the applied square wave and the pitch of the electrodes. Droplets with different volumes are tested by square waves of varied frequencies and amplitudes. The 1.0 µl droplet is successfully transported on a device with a loop of 24 electrodes continuously at a speed up to 23.6 mm s−1 when a 9 Hz square wave is applied.

Segregation in desiccated sessile drops of biological fluids

Abstract: It is shown here that concurrence between advection and diffusion in a drying sessile drop of a biological fluid can produce spatial redistribution of albumen and salt. The result gives an explanation for the patterns observed in the dried drops of the biological fluids.

Stable Perfluorosilane Self-Assembled Monolayers on Copper Oxide Surfaces: Evidence of Siloxy−Copper Bond Formation

Abstract: Formation of stable and hydrophobic self-assembled monolayers (SAMs) on an oxidized copper (Cu) surface has been accomplished via reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS). The perfluoroalkyl SAMs showed sessile drop static contact angles of more than 125° for pure water and stability against exposure to boiling water, boiling nitric acid solution, and warm sodium hydroxide solution for up to 30 min. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS) data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS-modified oxidized Cu surface is largely due to the formation of a siloxy−copper (−Si−O−Cu−) bond via a condensation reaction between silanol (−Si−OH) and copper hydroxide (−CuOH). The extremely hydrophobic and stable SAMs on oxidized Cu could have useful applications in...

Liquid metal/ceramic interactions in the (Cu, Ag, Au)/ZrB2 systems

Abstract: Wetting and spreading experiments on ZrB2 in contact with liquid Cu, Ag and Au have been performed by the sessile drop technique under a vacuum. The wetting and spreading characteristics and the interfacial reactions are discussed as a function of time and of the metal involved. The interfacial morphologies, analysed by optical microscopy, SEM and EDS show the presence of regular interfaces without macroscopic reaction layers. Gold, to a very large extent and copper are shown to give rise to extensive penetration along grain-boundaries, whereas silver neither wets nor penetrates. Interfacial diffusion/dissolution is taken into account and the consequent changes in liquid metal surface tension and wetting behaviours have been evaluated by means of thermodynamic calculations. Moreover, interfacial energetics at the atomistic level has been investigated by means of pseudopotential-based Density Functional Theory (DFT) technique. It is shown how the calculation of the ideal work of separation on the specific transition metal borides-molten metal systems can be used to interpret the wetting behaviour. Moreover, the dependence of the adhesion behaviour on the electronic structure at the interface and on the interface epitaxy and composition is also briefly discussed.

The nature of wetting on urban soil samples: wetting kinetics and evaporation assessed from sessile drop shape

Abstract: The understanding of soil water repellency in its complexity requires knowledge of the mechanisms leading to changes in surface characteristics. Wetting kinetics may serve as means to investigate the origin of soil water repellency, but have been scarcely investigated yet. We observe the wetting kinetics of soil samples from two locations via the time-dependent shape of sessile drop (TISED) at three temperatures. We show that drop penetration may be exceeded by evaporation for high water drop penetration time (WDPT). The time-dependent change of drop shape and apparent contact angle is explained by surface hydrophilisation including the change from Cassie-Baxter into Wenzel's state. We identify principal differences in the nature of water repellency between the two investigated locations: Only the samples from the former sewage field, Buch, lose most initial differences in wettability upon air drying and storage. Wetting of these samples requires an activation energy of 65–94 kJ mol−1, indicating chemical reactions as rate-limiting step. In contrast, wetting of the samples from the inner city park, Tiergarten, requires an activation energy of 42 kJ mol−1 for the repellent and 8–20 kJ mol−1 for the wettable samples, which suggests physico-chemical and physical processes as rate-limiting steps. Our study shows for the first time that the process of soil wetting can be monitored by TISED assessment, and that assessment of the temperature dependence of the wetting kinetics allows distinguishing between different natures of wetting and soil water repellency. It therefore represents a novel approach to investigate wetting processes. Combination of such approaches with spectroscopic investigations will help to deepen our understanding on possible causes of water repellency. They further help to understand the great variety of suggested causes of repellency and indicate locational material-specific effects rather than one general cause for water repellency. Copyright © 2007 John Wiley & Sons, Ltd.

Wetting and reactivity in Ni–Si/C system: experiments versus model predictions

Abstract: Wetting of vitreous carbon Cv by NiSi alloys is studied in high vacuum using the sessile drop and dispensed drop techniques. The role of reactions between NiSi and Cv (simple dissolution or SiC formation) on wetting, adhesion and mechanical behaviour of the interface is determined and discussed. The experimental results on reactive wetting are compared with the predictions of two different approaches proposed recently in order to explain the thermodynamics and kinetics of this type of wetting.

Validity of the "sharp-kink approximation" for water and other fluids

Abstract: The contact angle of a liquid droplet on a solid surface is a direct measure of fundamental atomic-scale forces acting between liquid molecules and the solid surface. In this work, the validity is assessed of a simple equation, which approximately relates the contact angle of a liquid on a surface to its density, its surface tension, and the effective molecule-surface potential. This equation is derived in the sharp-kink approximation, where the density profile of the liquid is assumed to drop precipitously within one molecular diameter of the substrate. It is found that this equation satisfactorily reproduces the temperature-dependence of the contact angle for helium on alkali metal surfaces. The equation also seems be applicable to liquids such as water on solid surfaces such as gold and graphite, based on a comparison of predicted and measured contact angles near room-temperature. Nevertheless, we conclude that, to fully test the equation's applicability to fluids such as water, it remains necessary to measure the contact angle's temperature-dependence. We hypothesize that the effects of electrostatic forces can increase with temperature, potentially driving the wetting temperature much higher and closer to the critical point, or lower, closer to room temperature, than predicted using current theories.

An analysis of the sensitivity of pendant drops and liquid bridges to measure the interfacial tension

Abstract: Drop shape techniques, such as axisymmetric drop shape analysis, provide accurate measurements of the interfacial tension from images of pendant drops for a wide variety of experimental conditions. However, these techniques are known to fail when dealing with nearly spherical drop shapes, which may occur, for instance, when working with interfaces between liquids of similar densities and/or under microgravity. We analyzed the advantages of using liquid bridges close to the minimum volume stability limit instead of pendant drops to measure the interfacial tension under different experimental conditions. First, the sensitivity of both configurations to a variation of the interfacial tension is studied numerically as a function of the volume for several Bond numbers B. The results indicate that a liquid bridge close to the minimum volume stability limit is generally more sensitive than a pendant drop of the same volume, especially for small values of the density difference across the interface and/or gravity. This suggests that the use of liquid bridges may extend the range of applicability of drop shape techniques. To explore this possibility, synthetic images of both pendant drops and liquid bridges were generated and then processed by TIFA-AI. The results demonstrated that the use of liquid bridges enhances the range of Bond numbers for which drop shape techniques work satisfactorily. More specifically, similar accuracy is obtained from both configurations for B ~ 10−1, while the use of liquid bridges yields much better results for B ~ 10−2. Finally, experiments were conducted to partially validate the analysis based on synthetic images. Good agreement was found between the values determined from the real and synthetic images.

Wetting of TiC by Al-Cu alloys and interfacial characterization.

Abstract: The wetting behavior and the interfacial reactions that occurred between molten Al-Cu alloys (1, 4, 8, 20, 33, and 100 wt% Cu) and solid TiC substrates were studied by the sessile drop technique in the temperature range of 800-1130 degrees C. The effect of wetting behavior on the interfacial reaction layer was studied. All the Al-Cu alloys react with TiC at the interface forming an extensive reaction layer. The interface thickness varied with the samples, and depends on the temperature, chemical composition of the alloy and the time of the test. Wetting increases with increasing concentration of copper in the Al-Cu alloy at 800 and 900 degrees C. In contrast, at higher temperature such as 1000 degrees C wetting decreases with increasing copper content. The spreading kinetics and the work of adhesion were evaluated. The high values of activation energies indicated that spreading is not a simple viscosity controlled phenomenon but is a chemical reaction process. The spreading of the aluminum drop is observed to occur according to the formation of Al4C3, CuAl2O4, CuAl2, TiCux mainly, leading to a decreases in the contact angle. As the contact angle decreases the work of adhesion increases with increasing temperature. Al-Cu/TiC assemblies showed cohesive fracture corresponding to a strong interface. However, using pure Cu the adhesion work is poor, and the percentage of cohesion work is also too low (27-34%).

Novel Characterization of Wetting Properties and the Calculation of Liquid-Solid Interface Tension(II)

Abstract: On the basis of the characterization and calculation of wetting property of the limited quid-solid interface, the difference between the deduced theoretical calculation solution and Y-G-G Empirical equation and Y-G-G-F-V Equation is contrasted; the theoretical formula is calculated and verified by the wetting test. The results showed that: exceptθ=0°and θ=180°, the calculated values of the theoretical solution in this study are always larger than the solutions of Y-G-G Equation and Y-G-G-F-V Empirical Equation, which showed that the solid surface tension calculated through Y-G-G Equation and Y-G-G-F-V Equation is smaller; It is found that, in the limitless liquid-solid interface wetting system, when the contact angles are large, the change in the solid surface tension has sensitive effect on the contact angle, the solid surface tension calculated with the theoretical equation is well consistent, which shows that the deduced formula can rightly reflect the natures of the solid surface tension; while the contact angle θ is smaller, the effect of the increase in the solid surface tension on the contact angle will become weaker, entering into the indeterminacy scope, and when the contact angle is less than 35°, the increase in the solid surface tension has the extremely weak effect on the contact angle, and the error in the wetting test is larger.

Water Contact Angle On Polyelectrolyte‐Coated Surfaces: Effects of Film Swelling and Droplet Evaporation

Abstract: The wettability by water of polyelectrolyte mono‐ and multilayers with different polycations or polyanions as the exposed layer is studied by contact angle measurements using the sessile drop technique. Measurements in a water‐saturated atmosphere and in ambient conditions [∼40% relative humidity (r.h.)] are made to study the effect of swelling and evaporation on the contact angle. It is found that these effects depend on the outermost layer of the polyelectrolyte coating. For a coating with Poly(styrene sulfonate) (PSS) as the exposed layer the water contact angle also indicates a change in the swelling state when the pre‐equilibrated film is directly in contact with liquid water. Droplet evaporation from polyelectrolyte coatings leads to a linear decrease of the contact angle with time. A significant enhancement of the droplet evaporation rate is found for a coating with Poly(diallyldimethylammonium bromide) (PDADMAC) as the exposed layer, which is attributed to the existence of a surface energy related...

Diffusion-limited reactive wetting: effect of interfacial reaction behind the advancing triple line

Abstract: Using the “dispensed drop” variant of the sessile drop technique, spreading kinetics of dilute Cu–Cr alloys on smooth vitreous carbon substrates are measured under helium microleak conditions. In this system, it is known that the drop spreading rate is controlled by diffusion of the reactive atom species (Cr) from the bulk liquid to the triple line, where wetting is induced by formation of an interfacial layer of chromium carbide. Microstructural characterization of rapidly cooled drops shows that growth of the interfacial reaction product layer continues behind the moving solid–liquid–vapor triple line. The spreading velocity is modeled by finite-difference numerical analysis of diffusion near the triple line in the presence of continued interfacial reaction, simplifying the growth rate as being constant and using realistic parameter values. We show that continued interfacial reaction explains the dependence of the triple line spreading rate on the instantaneous wetting angle that is observed in this system.

Influence of oxygen on the joining between copper and aluminium nitride

Abstract: Aluminium nitride has been developed for electronic ceramic packaging applications because of its high thermal conductivity and high electrical resistivity. To improve the heat dissipation at the metal/ceramic interface, a high quality bonding between the substrate and the copper conductor is needed. This process requires a previous step of AIN pre-oxidation of the substrates by oxygen gas at 1200 °C, in order to form a thin layer of Al 2 O 3 at the surface of AlN. The junction between Cu and the substrate is carried out at 1075 °C in controlled oxygen atmosphere which promote the oxidation of the copper and the formation of an eutectic phase which can form a strong junction with AlN via the layer of Al 2 O 3 . The goal of the current work is to study the influence of oxygen supplied by gaseous phase to form the exact amount of the eutectic phase needed to get a strong junction. First, in order to fix conditions for joining, the wetting behaviour of copper has been studied using the sessile drop method. The influence of oxygen brought by surrounding gas is given in terms of wettability of the liquid, interfacial tension and chemical reactions. According to previous results, copper foils and copper cylinders have been directly joined to AIN substrates. Interfacial reactions, mechanical and thermal properties have been investigated.

Chemical stability of nonwetting, low adhesion self-assembled monolayer films formed by perfluoroalkylsilanization of copper.

Abstract: A self-assembled monolayer (SAM) has been produced by reaction of 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS) with an oxidized copper (Cu) substrate and investigated by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, and contact angle measurement. FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. The perfluoroalkyl SAM on Cu is found to be extremely hydrophobic, yielding sessile drop static contact angles of more than 130° for pure water and a “surface energy” (which is proportional to the Zisman critical surface tension for a Cu surface with 0rms roughness) of 14.5mJ∕m2(nM∕m). Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60°C) solutions for at least 30min. Furthermore, no SAM degradation was observed when PFMS/Cu was ex...