Narrow Results

In this paper, the effects of surface wettability and topography on a droplet, which is driven by a body force to pass through grooved walls, are studied by using the multiphase lattice Boltzmann model. At small scale, the shape and velocity of the droplet were found to be strongly affected by the wettability and configuration of the wall. The drag on the droplet moving over grooved surfaces was found to decrease as the wall hydrophobicity increases. It was also found that the wettability decides whether the droplet fills or does not fill the whole grooves.

We investigate imbibition of a wetting liquid in power-law-shaped channels by numerical simulations with a two-phase lattice Boltzmann model. At short times, the dynamics of the imbibition follows diffusive scaling similar to that in uniform capillaries, but at longer times power-law behavior with exponent depending on the channel shape changes is observed. The observed behavior is in exact agreement with a recent theory of Reyssat and co-workers.¹

This paper was aimed to study of the wetting angle ($\mathrm{\Theta }$) of Sn–Ag–Cu, Sn–($3-x$)Ag–0.5Cu–($x$)Bi and Sn–($3-x$)Ag–0.5Cu–($x$)In ($x=0.5$, 1 and 2 in wt.%) Pb-free solder alloy systems at various temperatures (250, 280 and 310^∘C) on Cu substrate in Ar atmosphere. The new Sn–($3-x$)Ag–0.5Cu–xBi and Sn–($3-x$)Ag–0.5Cu$-$($x$) In systems, low Ag content quaternary Pb-free solder alloys, were produced by adding 0.5%, 1% and 2% Bi and In separately to the near-eutectic Sn-3wt.%Ag–0.5wt.%Cu (SAC305) alloy. The wetting angles of new alloys, Sn$-$2.5wt.%Ag$-$0.5wt.%Cu$-$0.5wt.%Bi (SAC-0.5Bi), Sn$-$2wt.%Ag$-$0.5wt.%Cu$-$1wt.%Bi(SAC-1Bi), Sn$-$1wt.%Ag$-$0.5wt.%Cu$-$2wt.%Bi(SAC-2Bi), Sn$-$2.5wt.%Ag$-$0.5wt.%Cu$-$0.5wt.%In (SAC-0.5In), Sn$-$2wt.%Ag$-$0.5wt.%Cu$-$1wt.%In (SAC-1In) and Sn$-$1wt.%Ag$-$0.5wt%.Cu$-$2wt.%In (SAC-2In) were measured by sessile drop method. Experimental results showed that additions of Bi and In separately to SAC305 resulted in a continuous decrease in the $\mathrm{\Theta }$ up to 1wt.% above which the $\mathrm{\Theta }$ value was increased and it is appeared that a correlation among the $\mathrm{\Theta }$, alloys compositions and the test temperatures exists which recommended an empirical model to estimate the $\mathrm{\Theta }$ at a given Bi and In content and temperature for a given alloy systems. The numerical model estimates the $\mathrm{\Theta }$ understandably well with the present work.

Accurately modeling two-phase flows involving the surfactant is of great significance for understanding the interface evolution dynamics due to the variation of surface tension. In this study, we proposed an Allen–Cahn-based lattice Boltzmann model for two-phase flows with soluble surfactants, in which a new force scheme from the view of continuous surface tension force scheme and the consistent wetting boundary condition are derived. Several numerical tests are conducted, and the results are compared with the model based on the previous potential form (PF) interfacial force scheme. The results indicate that our proposed model can model two-phase flows involving the soluble surfactants, even for high-density ratio situations. The distribution across the interface is accurately predicted. Our model also shows superior performance on two-phase dynamics involving surfactants, i.e. smaller spurious currents and larger range of contact angle. This model can be a favorable alternative for high-density-ratio two-phase flows involving soluble surfactants.

In the SU(3) pure gauge theory the high temperature deconfinement phase transition is first order. At the transition the confined phase coexists with three distinct deconfined phases. The interfaces separating the various phases show universal critical behavior when the confined phase completely wets the deconfined-deconfined interfaces. The critical exponents of complete wetting are derived from an interface solution of a Z(3) symmetric effective theory for the Polyakov loop. When complete wetting occurs in the pure glue theory the deconfinement transition proceeds via domain wall splitting. When quarks are present wetting disappears and the standard bubble nucleation scenario applies.

We investigate the adsorption of helium in alkali wedges at zero temperature within a fully unrestricted density–functional theory. We have found a sequence of shapes that includes one filling transition on the nonwettable Cs and one emptying transition for every alkali, as well as a threshold for metastable configurations where helium does not populate the wedge apex.

A continuum hydrodynamic model is presented for one-component liquid–gas flows on nonisothermal solid substrates. Numerical simulations are carried out for evaporative droplets moving on substrates with thermal gradients. For droplets in one-component fluids on heated/cooled substrates, the free liquid–gas interfaces are nearly isothermal. Consequently, a thermal singularity occurs at the contact line while the Marangoni effect due to interfacial temperature variation is suppressed. Through evaporation/condensation near the contact line, the thermal singularity makes the contact angle increase with the increasing substrate temperature. Due to this effect, droplets will move toward the cold end on substrates with thermal gradients. The droplet migration velocity is found to be proportional to the change of substrate temperature across the droplet. It follows that for two droplets of different sizes on a substrate with temperature gradient, the larger droplet moves faster and will catch up with the smaller droplet ahead. As soon as they touch, they coalesce rapidly into an even larger droplet that will move even faster.

When a binary mixture is quenched into the unstable region of the phase diagram, phase separation starts by spontaneous growth of long-wavelength concentration fluctuations ("spinodal decomposition"). In the presence of surfaces, the latter provide nontrivial boundary conditions for this growth. These boundary conditions can be derived from lattice models by suitable continuum approximations. But the lattice models can also be simulated directly, and thus used to clarify the conditions under which the Ginzburg–Landau type theory is valid. This comparison shows that the latter is accurate only in the immediate vicinity of the bulk critical point, if thermal fluctuations can also be neglected (true for the late stages of phase separation). In contrast, a local kinetic molecular field theory can take full account of nonlinearities and of rapid concentration variations, and thus has a much wider validity. This enables the detailed study of phase separation processes in thin films of solid binary alloys. However, the extension to spinodal decomposition in fluid binary systems (which can be simulated by brute force large scale molecular dynamics methods, of course) remains an unsolved challenge.

We present here an analysis of the regularity of minimizers of a variational model for epitaxially strained thin-films. The regularity of energetically-optimal film profiles is studied by extending previous methods and by developing new ideas based on transmission problems. The achieved regularity results relate to both the Stranski-Krastanow and the Volmer-Weber modes, the possibility of different elastic properties between the film and the substrate, and the presence of the surface tensions of all three involved interfaces: film/gas, substrate/gas, and film/substrate. Finally, geometrical conditions are provided for the optimal wetting angle, i.e. the angle formed at the contact point of films with the substrate. In particular, the Young–Dupré law is shown to hold, yielding what appears to be the first analytical validation of such law for a thin-film model in the context of Continuum Mechanics.

New compounds bearing two phosphate groups and two long chain (dodecyl) were prepared by two-step reaction: (i) phosphorylation of dodecanol with pyrophosphoric acid, (ii) reaction of dodecyl phosphate with N(CH₃)₄OH and 1,6-dibromo hexane. The effect of reaction variables like time and molar ratio of reactants on yield has also been reported. The 1:2:0.5 molar ratio of reactants (dodecyl phosphate, N(CH₃)₄OH, and Br(CH₂)₆Br, respectively) and 3 h duration resulted to give maximum yield of anionic gemini surfactants. The structure of synthesized surfactant was investigated by modern analytical techniques, viz. FT-IR, ¹H NMR, ¹³C NMR.

Amphipathic disodium phosphates were obtained by neutralization of free acids with sodium hydroxide and their surface active properties in aqueous solution were measured. These disodium phosphates possessed 77.3% anionic content and showed good water solubility. Foaming properties and wetting ability were also evaluated.

Advancing and receding contact angles below 5° have been obtained on PMMA surfaces with the implantation of argon and oxygen ions. The ion implantations were performed by means of the Immersion Plasma Ion Implantation (IPII) technique, a hybrid between ion beams and immersion plasmas. Characterization of treated PMMA surfaces by means of XPS and its combination with chemical derivatization (CD-XPS) have revealed the depletion of oxygen and the creation of dangling bonds, together with the formation of new chemical functions such as –OOH, –COOH and C=C. These observations provide a good explanation for the strong increase of the wetting properties of the PMMA surfaces.

In this study, a thermodynamic analysis is conducted to investigate the chemical effect, in terms of intrinsic contact angle (CA), on the superhydrophobic behavior. It is theoretically revealed that the essential effect of intrinsic CA is to promote the composite transition. In particular, a large intrinsic CA more than 90° is necessary for such transition. Furthermore, for a pillar system with an intrinsic CA smaller than 90°, composite states are not impossible but is thermodynamically unstable. Once composite states are achieved, the advancing or maximum CA always approaches 180° whether an intrinsic CA is larger or smaller than 90°. In addition, the role of intrinsic CA in the water-repellent or self-cleaning behavior such as contact angle hysteresis (CAH) and equilibrium CA has been discussed in detail.

An experimental pool boiling study was conducted using plain and nanoporous coated heater surfaces immersed in various working fluids: water, ethanol and HFE-7100. Pool boiling tests were performed on flat 1 cm × 1 cm heaters. Unlike in water, the critical heat flux (CHF) enhancement of the nanoporous coating seems to be less or marginal in ethanol and HFE-7100 at 1 atm. The reduced effect of the nanoporous coating in ethanol and HFE-7100 is believed to be due to the highly wetting nature of these fluids since no obvious difference in wettability is observed between nanoporous coated and uncoated surfaces through apparent contact angle measurement. Moreover, pressure effects were also investigated for the fluids mentioned above. For the nanoporous coated surface, CHF enhancement of the nanoporous coating appeared to be dependent on the test pressure, showing greater CHF enhancement at lower pressure. It is believed that this pressure dependent CHF enhancement behavior could be closely related to the bubble departure diameter. As pressure lowers, the departure bubble size increases and this allows the nanoporous coating to become more influential, even for the highly wetting fluids, in delaying local dry-out, which in turn results in increasing CHF enhancement.

Modification of surface chemical functionality of a variety of materials using the reactions of readily accessible diarylcarbenes permits the control of wetting behavior. When coupled with an additional chemical derivatization step, a wider range of surface polarity becomes available.

This paper is extracted from an opening address given at the workshop 'Wetting: from microscopic origins to industrial applications' (Giens, May 6–12, 2000). It discusses two special points a) the nature of line energies for a contact line b) the aging of emulsions.

Porous methylsiloxane gels with micrometer-scale bicontinuous structure have been prepared via sol-gel method accompanying spinodal decomposition in a variety of confined geometries. The resultant porous structure in a 2 dimensional mold has been examined closely utilizing the recent Laser Scanning Confocal Microscopy (LSCM). A drastic structural deformation due to preferential wetting by methylsiloxane-rich phase has lead to elongated columnar structure in the proximity of the surfaces. The area-averaged mean and Gaussian curvatures of spinodal interfaces were obtained by Parallel Surface Method (PSM). In the vicinity of the confining surfaces, values of the mean curvature were slightly deviated from that of the isotropic bicontinuous structure, reflecting linearly elongated features of methylsiloxane gel skeleton perpendicular to the surfaces. When a scaled thickness of the sample D/Λ_m becomes less than unity, wetting transition occurred. Here, Λ_m means characteristic length of bicontinuous structure of bulk gel prepared in a 3 dimensional free volume. The criterion for wetting transition, D/Λ_m < 1, was universal in all confined spaces.

We investigate the adsorption of helium in alkali wedges at zero temperature within a fully unrestricted density–functional theory. We have found a sequence of shapes that includes one filling transition on the nonwettable Cs and one emptying transition for every alkali, as well as a threshold for metastable configurations where helium does not populate the wedge apex.

Wet granulation is traditionally considered an empirical art. The chemical and allied industry faces range of problems, including large recycle ratios, poor quality control, surging and even the total failure of scale up from laboratory to full scale production. However, in recent years there has been a rapid advancement in understanding of the fundamental processes that control granulation behavior and product properties. This paper presents the current understanding of the key area of wet granulation process: nucleation.