Evaporation of an emulsion trapped in a yield stress fluid

The present work deals with emulsions of volatile alkanes in an aqueous clay suspension, Laponite, which forms a yield stress fluid. For a large enough yield stress (i.e. Laponite concentration), the oil droplets are prevented from creaming and the emulsions are thus mechanically stabilized. We have studied the evaporation kinetics of the oil phase of those emulsions in contact with the atmosphere. We show that the evaporation process is characterized by the formation of a sharp front separating the emulsion from a droplet-free Laponite phase, and that the displacement of the front vs. time follows a diffusion law. Experimental data are confronted to a diffusion-controlled model, in the case where the limiting step is the diffusion of the dissolved oil through the aqueous phase. The nature of the alkane, as well as its volume fraction in the emulsion, has been varied. Quantitative agreement with the model is achieved without any adjustable parameter and we describe the mechanism leading to the formation of a front.     

Evaporation of saline colloidal droplet and deposition pattern

The dynamic process of the evaporation and the desiccation of sessile saline colloidal droplets, and their final deposition are investigated. During the evaporation, the movement of the colloidal particles shows a strong dependence on the salt concentration and the droplet shape. The final deposition pattern indicates a weakened coffee-ring effect in this mixed droplet system. The microscopic observation reveals that as evaporation proceeds, the particle motion trail is affected by the salt concentration of the droplet boundary. The Marangoni flow, which is induced by surface tension gradient originating from the local evaporative peripheral salt enrichment, suppresses the compensation flow towards the contact line of the droplet. The inhomogeneous density and concentration field induced by evaporation or crystallization can be the major reason for various micro-flows. At last stage, the distribution and crystallization of Na Cl are affected by the colloidal particles during the drying of the residual liquid film.     

Vertical water distribution during the drying of polymer films cast from aqueous emulsions

We present a systematic study of the vertical uniformity of water distribution during the drying of waterborne colloidal films, testing the predictions of a Peclet number Pe defined for this system. Pe indicates the relative contributions of water evaporation and Brownian diffusion in determining the concentration profile in the vertical direction (i.e. normal to the substrate). When Pe < 1, the water concentration in films cast from an alkyd emulsion is found via magnetic-resonance profiling to be uniform with depth, which is consistent with expectations. When Pe > 1, a gradient in the water concentration develops, with less water near the interface with air. The water profiles reveal that the alkyd particles do not coalesce immediately upon contact in close-packing. At later times, a concentrated surface layer develops, but particles are not coalesced in this layer to form a continuous “skin”, but rather the structure is likely to be that of a biliquid foam. Received 20 March 2002 and Received in final form 12 June 2002     

Heat flux density in the region of droplet contact line on a horizontal surface of a thin heated foil

The evaporating water droplets on a horizontal heated substrate were experimentally studied. The constantan foil 25 μm thick with a size of 42×35 mm² was used as a substrate. The experiments were carried out with a single droplet or with an ensemble of two or three droplets on the foil. The temperature of the lower surface of foil was measured by an IR scanner. To determine the heat flux density at evaporation of liquid near the contact line, the Cauchy problem for the heat conduction equation was solved using the thermographic data. The results of calculations showed that the maximal heat flux density takes place in the region of the contact line and exceeds the average heat flux density from the entire surface of foil. This is explained by the heat inflow from the foil periphery to the droplet due to relatively high value of the coefficient of heat conductivity of the foil material and high evaporation intensity in the contact line region.     

DMRG studies on interchain coupling model for quasi-one-dimensional organic magnet

We introduce a solid-on-solid growth process which evolves by random deposition of dimers, surface diffusion, and evaporation of monomers from the edges of plateaus. It is shown that the model exhibits a robust transition from a smooth to a rough phase. The roughening transition is driven by an absorbing phase transition at the bottom layer of the interface, which displays the same type of critical behavior as the pair contact process with diffusion 2A↦3A, 2A↦. Received 14 October 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: Haye.Hinrichsen@physik.uni-wuppertal.de     

Comparing the first- and second-order theories of relativistic dissipative fluid dynamics using the 1+1 dimensional relativistic flux corrected transport algorithm

Focusing on the numerical aspects and accuracy we study a class of bulk viscosity driven expansion scenarios using the relativistic Navier–Stokes and truncated Israel–Stewart form of the equations of relativistic dissipative fluids in 1+1 dimensions. The numerical calculations of conservation and transport equations are performed using the numerical framework of flux corrected transport. We show that the results of the Israel–Stewart causal fluid dynamics are numerically much more stable and smoother than the results of the standard relativistic Navier–Stokes equations.     

Roughness and dynamics of a contact line of a viscous fluid on a disordered substrate

We have studied the roughness and the dynamics of the contact line of a viscous liquid on a disordered substrate. We have used photolithographic techniques to obtain a controlled disorder with a correlation length ξ = 10μm. Liquids with different viscosity were used: water and aqueous glycerol solution. We have found that the roughness W of the contact line depends neither on the viscosity nor on the velocity v of the contact line for v in the range 0.2-20μm/s. W is found to scale with the length L of the line as L ^ζ with a roughness exponent ζ = 0.51±0.03. This value is similar to the one obtained with superfluid helium. In the present experiment, we have checked that the motion of the contact line is actually overdamped, so that the phenomenological equation first proposed by Ertas and Kardar should be relevant. However, our measurement of ζ is in disagreement with the predicted value ζ = 0.39. We have also analyzed the avalanche-like motion of the contact line. We find that the size distribution does not follow a power law dependence. Received 18 April 2002     

Optimal sample preparation for nanoparticle metrology (statistical size measurements) using atomic force microscopy

Optimal deposition procedures are determined for nanoparticle size characterization by atomic force microscopy (AFM). Accurate nanoparticle size distribution analysis with AFM requires non-agglomerated nanoparticles on a flat substrate. The deposition of polystyrene (100 nm), silica (300 and 100 nm), gold (100 nm), and CdSe quantum dot (2–5 nm) nanoparticles by spin coating was optimized for size distribution measurements by AFM. Factors influencing deposition include spin speed, concentration, solvent, and pH. A comparison using spin coating, static evaporation, and a new fluid cell deposition method for depositing nanoparticles is also made. The fluid cell allows for a more uniform and higher density deposition of nanoparticles on a substrate at laminar flow rates, making nanoparticle size analysis via AFM more efficient and also offers the potential for nanoparticle analysis in liquid environments.     

Evaporation of liquid droplets from a surface of anodized aluminum

The results of study of evaporation of water droplets and NaCl salt solution from a solid substrate made of anodized aluminum are presented in this paper. The experiment provides the parameters describing the droplet profile: contact spot diameter, contact angle, and droplet height. The specific rate of evaporation was calculated from the experimental data. The water droplets or brine droplets with concentration up to 9.1 % demonstrate evaporation with the pinning mode for the contact line. When the salt concentration in the brine is taken up to 16.7 %, the droplet spreading mode was observed. Two stages of droplet evaporation are distinguished as a function of phase transition rate.     

Growth and Auger electron spectroscopy characterization of donut-shaped ZnO nanostructures

Zinc oxide (ZnO) nanodonuts have been obtained by vapor phase transport process utilizing a mixture of ZnO, graphite and erbium oxide powder as the evaporation source. ZnO nanodonuts prepared under various thermal processes indicate that ZnO nanodonuts start forming during the initial thermal ramp up stage. A subsequent holding of the growth temperature at 1000 °C causes the nanodonut to evolve into perfectly donut-shaped nanostructure. Additional deposition of ZnO on top of the nanodonut during the holding of the furnace temperature at 1000 °C result in partially filled nanodonuts or hemispherical nanostructures, or donuts that are completely buried beneath ZnO film. Auger electron spectroscopy depth profile analysis indicates that the deposited ZnO film is stoichiometric, whereas the nanodonuts and the completely filled hemispherical nanostructures are porous and are oxygen deficient. The volume density of the nanodonut is estimated to be 20% that of the background ZnO film.     

Development of the diffusion-inertia model of particle deposition in turbulent flows

There is presented a modification of the diffusion-inertia model that describes the distribution and deposition of low-inertia particles in turbulent near-wall flows. For the transport equation of the dispersed phase concentration, there is proposed a new wall function that takes into account the nonequilibrium effects and nonlocality of the turbulent transport of the dispersed phase in the near-wall zone caused by the particles’ inertia. This allowed widening the applicability limits of the diffusion-inertia model even for particles with a relaxation time with a magnitude of several hundred. The calculation results for the rate of the particles’ deposition from the turbulent flow to the walls in a round pipe are in good accord with the literature experimental data and the data of direct numerical simulation.     

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic,biological and nanoparticle thin films: a review

The matrix-assisted pulsed laser evaporation (MAPLE) technique offers an efficient mechanism to transfer soft materials from the condensed to the vapor phase, preserving the versatility, ease of use and high deposition rates of the pulsed laser deposition (PLD) technique. The materials of interest (polymers, biological cells, proteins, …) are diluted in a volatile solvent. Then the solution is frozen and irradiated with a pulsed laser beam. Here, important results of MAPLE deposition of polymer, biomaterials and nanoparticle films are summarized. Finally, the MAPLE mechanism is discussed. A review of experimental and theoretical works points out that the simple model of individual molecule evaporation must be abandoned. Solute concentration, solubility, evaporation temperature of solvents, laser pulse power density and laser penetration depth emerge as important parameters to explain the morphology of the MAPLE-deposited films.     

Pinning-depinning of the contact line on nanorough surfaces

We study the pinning-depinning phenomenon of a contact line on a solid surface decorated by a random array of nanometric structures. For this purpose, we have investigated the contact angle hysteresis behaviour of six different wetting and non-wetting fluids with surface tensions varying from 25 to 72mN m^-1. For low values of the areal density of defects φ_d, the hysteresis H increases linearly with φ_d indicating that “individual” defects pin the contact line. Then, from a given value of φ_d, the hysteresis H becomes to decrease with increasing φ_d, indicating a new kind of collective depinning. These two regimes were observed for all fluids used. In both cases, our experimental results are compared with the theoretical predictions for contact angle hysteresis induced by single or multiple topographical defects. We ascribe the decrease of H to the formation of cavities along the wetting front.     

On integrable singularities and apparent contact angles within a classical paradigm Partial and complete wetting regimes with or without phase change

Far from claiming any ultimate resolution of the contact line paradoxes, we draw the reader’s attention to the fact that a somewhat controversial paradigm, originally employed by de Gennes and collaborators, actually appears both to be quite reasonable at its foundations and to lead to physically consistent final results in a wide variety of situations. Curiously enough, while containing a singularity in itself, the approach nonetheless renders the classical contact-line singularities — both hydrodynamic and thermal — integrable, in particular as far as several quantities of interest are concerned. It is also readily applicable to quite a few situations: from equilibrium shapes and moving contact lines of a non-volatile liquid, to cases with evaporation into (and even condensation from, although it is not studied here) either a pure-vapor or an inert-gas atmosphere. The paradigm actually consists in an approach involving both the (positive or negative) spreading coefficient and the disjoining pressure in the form of a positive inverse cubic law, a conceptual framework that most notably describes structures with truncated precursor films on a macroscopically bare solid surface. Whether or not the remaining integrable divergences at molecular scale can truly be considered as “benign” has to be discussed on the basis of more involved mesoscopic or microscopic approaches, quite outside the scope of the present study.     

Correlated electron current and temperature dependence of the conductance of a quantum point contact

We investigate finite temperature corrections to the Landauer formula due to electron–electron interaction within the quantum point contact. When the Fermi level is close to the barrier height, the conducting wavefunctions become peaked on the barrier, enhancing the electron–electron interaction. At the same time, away from the contact the interaction is strongly suppressed by screening. To describe electron transport we formulate and solve a kinetic equation for the density matrix of electrons. The correction to the conductance G is negative and strongly enhanced in the region 0.5 × 2e²/h ≤ G ≤ 1.0 × 2e²/h. Our results for conductance agree with the so-called “0.7 structure” observed in experiments.     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

Driven lattice gas with nearest-neighbor exclusion: shear-like drive

We study the lattice gas with nearest-neighbor exclusion on the square lattice and Kawasaki (hopping) dynamics, under the influence of a nonuniform drive, via Monte Carlo simulation. The drive, which favors motion along the +x direction and inhibits motion in the opposite direction, varies linearly with y. (The boundaries along the drive direction are periodic, so that the system is not described by an equilibrium Gibbs distribution.) As in the uniformly driven case [R. Dickman, Phys. Rev. E 64, 16124 (2001)], the onset of sublattice ordering occurs at a lower density than in equilibrium, but here an unexpected feature appears: particles migrate out of the high-drive region. For intermediate system sizes (L ≃100), the accumulation of particles is sufficient for the low-drive region to become ordered at a global density of about 0.3. Above this density we observe a surprising reversal in the density profile, with particles accumulating to the high-drive region, due to jamming. For larger systems (L≥200) particles quickly jam in the high-drive region, as occurs under uniform drive, and the accumulation of particles in the low-field region is severely reduced.     

Preparation of transparent conducting ZnO:Al coating using hot wall deposition technique

Aluminium-doped zinc oxide thin films were grown on glass substrates using hot wall deposition technique. The method involved evaporation of high purity ZnO and Al from respective sources surrounded by alumina cylinder held at high temperature. The additional thermal energy supplied by the hot wall to the particles colliding on it helps in its migration resulting in highly uniform films. XRD studies show the growth of c-axis oriented near single phase ZnO films having about 90% transmittance in the visible range and resistivity of the order of 10⁻⁴Θ cm.     

Experimental investigation of heat transfer in a rivulet on the inclined foil

Heat transfer at rivulet water flow over the constantan foil with the length of 80 mm, width of 35 mm, and thickness of 25 mm was studied experimentally. The foil surface temperature was measured by an IR-scanner. Distributions of heat flux density on the surface of the foil, where the liquid flowed, were obtained. To determine the heat flux density from the foil to liquid near the contact line, the Cauchy problem was solved for the stationary heat equation using the thermographic data. Calculation results showed that the maximal heat flux occurs in the area of the contact line and exceeds the average heat flux from the entire foil surface by several times. This is explained by the influx of heat from the periphery of foil to the rivulet due to the relatively high value of heat conductivity coefficient of the foil material and high evaporation rate in the region of the contact line.