Viscoelasticity and Kinetics of Wetting on Rubber

The kinetics of spreading of a liquid drop is usually controlled by conversion of capillary potential energy into viscous dissipation within the liquid when the solid is rigid. However, if the solid is soft, a “wetting ridge” near the solid/liquid/vapour triple line can also be a dissipative sink as the wetting front moves. As a consequence, the kinetics of wetting of rubber may be controlled essentially by viscoelastic losses in the polymer rather than by viscous losses in the liquid drops. Therefore, a direct analogy between the kinetics of wetting and adhesion, respectively, for a liquid and a solid on an elastomeric substrate has been recently proposed. In this paper, the superposition of viscoelastic braking and moderate rubber swelling in the drop spreading phenomenon is considered.     

Ni-56Si合金对SiC陶瓷的润湿和铺展

采用座滴法考察了不同温度和气氛下Ni-56Si合金对SiC陶瓷的润湿和铺展行为。分析了固化后的液滴/SiC衬底界面和液滴表面的微结构和相组成。结果表明:该润湿体系于1350℃下在真空、1个大气压的静态或流动的Ar+5%(体积分数)H2的混合气3种气氛中均可获得优良的润湿性。在真空条件下,其铺展最快达到平衡,1350℃时的润湿性明显优于1100℃和1200℃,且在1100℃时的铺展过程与1350℃和1200℃有显著不同,即存在2种不同的铺展机制,这与SiC表面的润湿壁垒(SiO2层)的消除动力学有关。界面微结构分析进一步证实该系统的非反应润湿特性。     

Assessment of the Wettability of Porous Electrodes for Lithium-Ion Batteries

The wettability of lithium cobalt oxide (LiCoO₂) and mesocarbon microbead electrodes in nonaqueous electrolyte is analyzed by a mathematical model of capillary liquid movement. Results show that wetting in the LiCoO₂ electrodes is difficult as compared with the MCMB electrodes at the same electrolyte composition. Wetting in the porous electrodes is controlled mainly by electrolyte penetration and spreading in pores. Electrolyte penetration is determined by viscosity. On the other hand, electrolyte spreading is controlled by surface tension. Organic solvent composition and lithium salt concentration may influence the wettability of porous electrodes due to changes in the viscosity and surface tension of the electrolyte. Increasing the amount of EC and/or lithium salts can cause poorer electrolyte spreading and penetration. Furthermore, careful pressure control has a positive effect on increasing the surface area of the solid–liquid interface. AC impedance data show that batteries with vacuuming prior to electrolyte filling may reach a maximum wetting in a few hours. If no vacuuming is applied, a few days are required to obtain sufficient wetting.     

Direct measurement of molten poly(ethylene terephthalate) contact angles on single aminopropylsilane coated glass fibers

The kinetics of spreading of molten poly(ethylene terephthalate) (PET) on aminopropylsilane (APS) treated glass fiber surfaces has been studied. The modified Wilhelmy wetting method was used for contact angle determination with small diameter fibers (10-100 μm) used as vertical wetting probes. It is shown that the small meniscus dimensions accelerates wetting, and viscous relaxation effects can be kept to a minimum, so that purely surface effects can be studied in terms of the equilibrium contact angle at elevated temperatures. Experiments were performed using single fibers coated with a 'reactive' coupling agent (APS) in this laboratory. Although complete spreading of PET at 270°C was not initially observed, zero contact angle was eventually obtained due to interfacial chemical reaction between APS and PET. The suggestion that complete spreading is driven by the interfacial reaction was supported by a study of glass fibers coated with a non-reactive silane. In that case, a large initial contact angle was observed but zero contact angle was never obtained. The equilibration rate to = 0° was much faster on bare glass than APS coated glass for PET at 270°C because complete spreading is thermodynamically favorable on bare glass. Data for commercial silane treated 10 μm fibers in terms of the wetting rate in molten PET at 270°C were comparable to that for the APS coated fibers.     

Spreading of Polydimethylsiloxane Drops on Solid Horizontal Surfaces

Effect of the volume of drops, surface energy and roughness of substrate together with temperature and viscosity on the spreading velocity of polydimethylsiloxane (PDMS) drops on solid horizontal surfaces was studied. Spreading velocity was shown to grow with decreasing drop volume, the effect being more pronounced at high viscosities of polymer. The deviation of shape of the spreading drop from that of a spherical segment is more pronounced the higher the surface energy of substrate, the higher the polymer viscosity and the smaller the drop volume. Spreading on a rough surface is slower than on a smooth one owing to the energy barrier created by surface inhomogeneities: the barrier is to be overcome by the spreading liquid. Based on the experimental results a mechanism of spreading of polymer drops is proposed. Changes in potential energy of a drop and in the free surface energy of the system during spreading were compared, allowing a theoretical evaluation of the influence of gravity on the spreading velocity of drops. A theoretical analysis of spreading kinetics of viscous drops is given. The equation proposed agrees well with the experimental results at 90° > θ > 0°.     

Wetting and interfacial phenomena of Ni‐Ta alloys on CVD‐SiC

When designing high‐temperature brazing processes for ceramic materials, the interfacial phenomena between the liquid media and the adjoining surfaces must be known to design reliable joints. In order to assess the feasibility of using Ta‐containing alloys for high‐temperature brazing of SiC‐based composites, the wetting of molten Ni‐Ta alloys on ultra‐pure chemical vapor deposited (CVD) SiC substrates has been investigated using the sessile drop technique. For all of the compositions and experimental conditions, good wetting was observed with fast spreading. The interfacial behavior is determined by the competition between the typical interfacial phenomena of the pure elements; specifically, either the dissolution of the ceramic phase by Ni or the formation of a new interfacial layer, TaC, by reaction between Ta and CVD‐SiC, which prevails depends on the relative amount of the single element in the alloy.     

The missing parameter in rheology: hidden solid‐like correlations in viscous liquids,polymer melts and glass formers

BACKGROUND: Determination of dynamic relaxation consists of measuring the viscous and the elastic components of a material by generally applying a small (oscillatory) deformation. The shear stress is transmitted to the material via contact with a substrate. Dating at least back to Stokes, the no‐slip boundary condition between the fluid and the substrate is supposed to be fulfilled during this measurement. We show that the viscoelastic parameters of fluids are usually not determined under no‐slip boundary conditions and do not originate from the first linear regime. Viscous and viscoelastic fluids (entangled and unentangled polymers, glass formers) measured under no‐slip conditions exhibit a fundamentally different response with a dominant terminal solid‐like response. RESULTS: We show that the terminal behaviour of fluids such as liquid polymers or glass formers measured at the sub‐millimetre scale and far above the glass transition is not viscous but solid‐like. Instead of a viscoelastic behaviour scaling as ω and ω² (ω is the frequency) for the viscous and the elastic moduli, respectively, the dynamic response is simplified; for low gap thickness, both viscous and elastic moduli are invariant with respect to the frequency (with the elastic modulus being larger than the viscous modulus) and enhanced by two to four orders of magnitude compared to the conventional viscoelastic response. Over a critical strain amplitude, the solid‐like response decreases and is progressively replaced by the conventional viscoelastic behaviour. We discuss the implications of this observation and reconsider the assumptions inherent to a rheology measurement. CONCLUSION: The identification of so far neglected macroscopic elasticity in the fluidic state far above the glass transition temperature in entangled and unentangled polymers and glass formers shows that the liquid state is dominated by long range intermolecular interactions. This information is fundamental to understand and to foresee dynamic behaviour; it sheds further light on nonlinear phenomena such as large time scale relaxations, rheo‐thinning, violation of the no‐slip boundary condition and spectacular shear‐induced instabilities (spurt effect, ‘shark‐skin’ instabilities, gross melt fracture, etc.) that are unpredictable in the frame of the conventional viscoelastic approach. It also implies that the viscoelastic times (reptation, Rouse) in polymers are not the longest relaxation times of these materials. Copyright © 2009 Society of Chemical Industry     

Spreading of a non-Newtonian liquid drop over a horizontal plane

The spreading of a drop of non-Newtonian (power-law) liquid over a horizontal solid substrate is analyzed theoretically through energy approach method in the case of complete wetting. In this approach we have used the physical and geometrical reasoning and finally obtained a relation between the rate of spreading and bottom radius of the drop. It is shown that spreading rate of shear thickening liquid is more than that of a Newtonian liquid while shear thinning liquid is having slower rate than the latter one.     

Profiles of Spreading Sessile Drops of Viscous Polymer Melts

The profile or shape of spreading drops of viscous polymer melts has always raised some questions regarding the basic forces inherent in and controlling the wetting phenomenon. The occurrence and nonoccurrence of “projecting feet” for spreading sessile drops has puzzled experimentalists for some time. Recent work for a homologous polymer series, differing only in molecular weight and molecular weight distribution and examined over a wide temperature range, has emphasized that an advancing projecting foot does occur and is dependent upon the molecular weight of the polymer material. The projecting feet can be interpreted as an effect resulting from the viscoelastic response of the material which can occur when the molecular weight of the polymer is higher than the characteristic critical molecular weight of entanglement of the material. For low molecular weight polymers the strong interfacial forces are not impeded by the bulk viscous response of the material whereas the present evidence indicates that, for high molecular weight material, the elastic or entangled component of the polymer may retard complete bulk flow or the redistribution of the polymer in harmony with the advancing interface. Previous inconsistencies in the literature regarding the observations of projecting feet may be explained by this model.     

The liquid spreading on porous solids: Dual action of pores

Experimental observation of the dual effect of pores in liquid spreading over porous substrates, whereby liquid movement is facilitated as well as restricted is presented based on spreading of micro-liter-sized liquid drops on substrates that have saturated (filled) millimeter-sized pores. The drops were put on porous and nonporous parts of solid substrate. The substrate was then rotated in vertical direction and the resulting motion of drops was recorded by a video camera. The analysis of the recorded images revealed that depending on whether the drop edge is moving toward the pore or away from the pore, the pore acts as accelerator or brake for the drop edge. This dual nature of the saturated pores can be ascribed to the attraction between the liquid in the drop and the liquid inside the pore. Qualitative changes in the morphology of the drop as it slides over saturated pores are also presented to highlight the process. This dual effect of pores is expected to play a major role in processes such as flow through a trickle bed of porous catalyst where it manifests itself in increased wetting efficiencies as well as pronounced hysteresis [Khanna, R., Nigam, K.D.P., 2002. Partial wetting in porous catalyst: wettability and wetting efficiency. Chemical Engineering Science 57, 3401-3405; Maiti et al., 2004. Enhanced liquid spreading due to porosity. Chemical Engineering Science 59, 2817-2820; 2005. Trickle-bed reactors: Porosity induced hysteresis. Industrial and Engineering Chemistry Research, in press.].     

Wetting,reactivity, and phase formation at interfaces between Ni–Al melts and TiB2 ultrahigh‐temperature ceramic

The wetting, reactivity, and phase formation at the liquid Ni–Al/TiB₂ ceramic interfaces have been investigated at the temperatures close to the Ni–Al liquidus line. The wetting kinetics has been studied by the sessile drop technique utilizing liquid drop dispension and high‐speed high‐resolution video imaging. It is established that the wetting behavior changes from a nonreactive for the Al‐rich melts to a dissolution‐reactive for the Ni‐rich melts. For the Ni concentration ≥40 at.%, TiB₂ precipitates are found in the solidified Ni–Al droplets after the high‐temperature interaction of the melts with TiB₂ substrates. Besides, new (Al,Ti)Ni₃ and (Al,Ti)₂Ni₂₁B₆ phases are formed due to dissolution of TiB₂ ceramic in Ni‐rich melts and subsequent solidification.     

Anisotropic Wetting of Liquids on Finely Grooved Surfaces

A photolithographically-prepared, parallel-grooved surface on silica has been employed as a model to study the influence of roughness on the spreading equilibrium of liquid drops. The equations generated by Oliver, Huh and Mason for cylindrically shaped drops were extended to account for wetting by liquid crystals. The observed drop shapes were dependent upon surface roughness. The equilibrium contact angles on a smooth surface can be calculated from the roughness, contact angles both parallel and perpendicular to the grooves, and the drop shape. Reasonably good agreement with experimental contact angles was obtained.     

MASS TRANSFER WITH CHEMICAL REACTION IN PARTIALLY WETTED FLAT PLATE CATALYST PELLETS: RIVULET FLOW ANALYSIS

Mass transfer with chemical reaction is analyzed in a system formed by a flat plate solid catalyst, partially wetted by a flowing rivulet of a liquid in contact with a stagnant pure gas. The paper solves the fluid dynamic problem of the liquid phase first, and afterwards incorporates the mass transfer and the chemical reaction. The system is assumed to be isothermal and at steady state, with a first order kinetics whose limiting reactant is in the gas phase. This work studies the influence of the gas-liquid surface tension, the liquid reactant flow rate, the liquid viscosity and the angle of inclination of the solid, upon the wetting factor. The model proposed also predicts the effect of these parameters and the Thiele modulus on the overall effectiveness factor and the molar flux of the limiting gaseous reactant at the catalytic solid-liquid interface in a direct way. This approach makes the wetting factor a non-manipulated variable.     

Capillary Driven Motion: The „Crawling“ Drop and a Possible Link with Phagocytosis

A sessile drop at equilibrium on an ideal solid surface satisfies both Laplace's equation for the meniscus shape and Young's equation along the triple line. If, however, a single, small, energetic heterogeneity exists near the triple line, although meniscus equilibrium is assured, Young's equation is unsatisfied and a net force acts on the drop, causing it to „crawl“ in order to engulf the inhomogeneity (of higher surface free energy). A dynamic energy balance between excess capillary energy and viscous dissipation governs the crawling motion. Similarities with phagocytosis are apparent, although the latter process generally occurs in a liquid medium without the presence of a solid substrate. The behaviour of a drop is, therefore, extended to that of a model cell suspended in a liquid medium. Although very much simplified, the system is modelled as an essentially spherical cell possessing a pseudopodium in contact with a small, spherical contaminant. By analogy with the crawling drop, a dynamic balance between excess interfacial free energy and viscous dissipation is established to estimate the time of engulfment, or phagocytosis, of the contaminant.     

Parallel hydrogenation for the quantification of wetting efficiency and liquid–solid mass transfer in a trickle‐bed reactor

A novel method for the measurement of wetting efficiency in a trickle‐bed reactor under reaction conditions is introduced. The method exploits reaction rate differences of two first‐order liquid‐limited reactions occurring in parallel, to infer wetting efficiencies without any other knowledge of the reaction kinetics or external mass transfer characteristics. Using the hydrogenation of linear‐ and isooctenes, wetting efficiency is measured in a 50‐mm internal diameter, high‐pressure trickle‐bed reactor. Liquid–solid mass transfer coefficients are also estimated from the experimental conversion data. Measurements were performed for upflow operation and two literature‐defined boundaries of hydrodynamic multiplicity in trickle flow. Hydrodynamic multiplicity in trickle flow gave rise to as much as 10% variation in wetting efficiency, and 10–20% variation in the specific liquid–solid mass transfer coefficient. Conversions for upflow operation were significantly higher in trickle‐flow operation, because of complete wetting and better liquid–solid mass transfer characteristics. © 2010 American Institute of Chemical Engineers AIChE J, 2011.     

Effect of viscosity in ternary polymer blends displaying partial wetting phenomena

Partial wetting in a ternary polymer blend is the thermodynamic state where all three phases meet at a three-phase line of contact. Pickering emulsions, where solid particles situate at the interface of two other phases is a classic example of this state. This paper studies the presence of partial wetting in PE/PP/PS and in PE/PP/PC ternary polymer blends and examines, in particular, the influence of polyethylene viscosity on PS droplet formation at the PE/PP interface. Quantitative analysis of PS droplet growth and coverage at the PE/PP interface during static annealing were obtained by image analysis. A new approach was established to estimate the co-continuous PE/PP coarsening rate and was found to be in agreement with previous studies. In this work it is shown that the polyethylene viscosity can be of significant importance in ternary partial wetting when the interfacial driving force for partial wetting is weak and viscosity directly affects the quantity and size of PS droplets at the interface during annealing. The equilibrium between droplet stability at the interface, as predicted by spreading theory, and the interfacial mobility generated by coarsening determines the PS droplet size and surface coverage at the PE/PP interface.A ternary PE/PP/PC system, which displays a strong partial wetting driving force, was also investigated. The morphology of the blend system studied demonstrated a clear dominance of partial wetting over complete wetting.     

Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal

In order to propel the application of the developed CuNi-Xwt%Ti active filler metal in AlN brazing and get the universal reactive wetting mechanism between liquid metal and solid ceramic, the reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal were investigated. The results indicate that, with the increasing Ti content, surface tension for liquid CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval, which influence the wetting behavior on AlN ceramic obviously. CuNi/AlN is the typical non-reactive wetting system, the wetting process including rapid wetting stage and stable stage. The wettability is depended on surface tension of the liquid CuNi filler metal completely. However, the wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages, which are rapid wetting stage decided by surface tension, slow wetting stage caused by interfacial reaction and stable stage. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ₀ at rapid wetting stage, the reduced free energy caused by interfacial reaction at slow wetting stage plays the decisive role in influencing the final wettability.     

涂料液体对塑料底材润湿行为的热力学研究

从热力学角度对涂料液体在塑料底材的附着润湿、浸透润湿、铺展润湿行为进行了分析,定性地研究了塑料表面Gibbs能与粗糙因子、涂料液体的表面Gibbs能对塑料-涂料液体界面润湿行为的影响,并用实验进行了验证。