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At least 1,063 records · Page 59Linked to original sources
PubMed2003

Three-dimensional lattice-Boltzmann model of van der Waals fluids.

A three-dimensional lattice-Boltzmann model is developed for the simulation of nonideal fluids under static and flow conditions. The van der Waals formulation of quasilocal thermodynamics for nonuniform fluids is used, and the interfacial stress tensor for nonideal fluids appears explicitly in the hydrodynamic equations. The continuity and flow equations are fully recovered, and Galilean invariance is restored through appropriate manipulations of the pressure tensor. Although applied here to the D3Q15 lattice, the methodology of Galilean restoration can be easily modified for use with other three-dimensional lattices as well. The Laplace law and Gibbs-Thomson equations are satisfied with excellent accuracy by the model, as demonstrated by droplet equilibrium simulations. Spinodal decomposition and droplet coalescence simulations are also carried out, revealing a direct proportionality of the characteristic times to the viscosity, as expected. A wettability adjustment was made possible through the prescription of a chemical potential profile along the fluid-wall interface, and used for the simulation of droplet formation from a conical orifice.

Journal Article↗
PubMed2003

Wetting behavior of associating binary mixtures at attractive walls: a lattice Monte Carlo study.

The lattice gas model is used to study the effects of molecular association on the wettability of surfaces with attractive walls by binary symmetric associating mixtures. The model assumes that the adsorbate particles occupy a regular cubic lattice of sites and that the interactions between adsorbate particles involve only the first nearest neighbors. The energies of interaction between the pairs of like particles are the same, while the only interaction between a pair of unlike particles is due to association. Only the formation of dimers is allowed and the energy of association is finite. The particles are subject to the surface, van der Waals-like potential, assumed to be the same for both components. The model is studied with the help of the Monte Carlo simulation method in the grand canonical ensemble. Only the ground state properties are treated analytically. It is demonstrated that, in general, molecular association hinders wetting. In particular, in the systems with nonzero wetting temperature, the increase of the association energy leads to the increase of the wetting temperature and for sufficiently high energy of association the mixture does not wet the surface at all. When the system is expected to exhibit complete wetting at the ground state, the film formed by strongly associating mixtures wets the surface only at sufficiently low temperatures, below the dewetting temperature. It is demonstrated that the dewetting temperature increases with the strength of the surface potential as well as with the increase of the association energy.

Journal Article↗
PubMed2003

Immiscible displacements in rough-walled fractures: competition between roughening by random aperture variations and smoothing by in-plane curvature.

Phase structure during capillary displacement of fluid phases within rough-walled fractures is controlled by the competition between random aperture variability which tends to roughen the interface and in-plane curvature which tends to smooth it. We show that the phase structure and corresponding areal saturation at the end of displacement depend primarily on the ratio of two dimensionless parameters: one that controls roughening (the coefficient of variation of the aperture field, delta) and another that controls smoothing (the curvature number C, which weighs the mean influences of aperture induced and in-plane curvature). Interestingly, for C/delta above approximately 0.5, areal saturation for wetting and nonwetting invasion first diverges and then converges to create an envelope, whose width increases with delta. This nonunique behavior with respect to wettability is fundamental to capillary displacements in rough-walled fractures and is due to an asymmetry in capillary competition in wetting versus nonwetting invasions.

Journal Article↗
PubMed2005

Pore-scale investigation of viscous coupling effects for two-phase flow in porous media.

Recent studies have revealed that viscous coupling effects in immiscible two-phase flow, caused by momentum transfer between the two fluid phases, can be important in porous medium systems. In this work, we use a three-dimensional parallel processing version of a two-fluid-phase lattice Boltzmann (LB) model to investigate this phenomenon. A multiple-relaxation-time (MRT) approximation of the LB equations is used in the simulator, which leads to a viscosity-independent velocity field. We validate our model by verifying the velocity profile for two-phase flow through a channel with a square cross section. We then simulate co-current flow through a sphere-pack porous medium and obtain correlations of the relative permeabilities as a function of capillary number, wettability, and the fluid viscosities. The results are qualitatively consistent with experimental observations. In addition, we calculate the generalized permeability coefficients and show that the coupling coefficients are significant and the matrix is nonsymmetric. We also find a strong correlation between the relative permeability and interfacial area between fluids, indicating that both the common extension of Darcy's Law and the generalized formulation accounting for viscous coupling effects do not provide adequate insight into two-phase flow processes in porous media. This work lends additional support for the hypothesis that interfacial area is a key variable for multiphase flow in porous medium systems.

Algorithms↗
PubMed2006

Electrowetting films on parallel line electrodes.

A lubrication analysis is presented for the spreading dynamics of a high permittivity polar dielectric liquid drop due to an electric field sustained by parallel line electrode pairs separated by a distance R(e). The normal Maxwell stress, concentrated at the tip region near the apparent three-phase contact line, produces a negative capillary pressure that is responsible for pulling out a thin finger of liquid film ahead of the macroscopic drop, analogous to that obtained in self-similar gravity spreading. This front-running electrowetting film maintains a constant contact angle and volume as its front position advances in time t by the universal law 0.43R(e)(t/T(cap))1/3, independent of the drop dimension, surface tension, and wettability. T(cap)=pi(2)mu(l)R(e)/8(epsilon0epsilonl)V2 is the electrocapillary time scale where mu(l) is the liquid viscosity, epsilon0epsilonl the liquid permittivity, and V the applied voltage. This spreading dynamics for the electrowetting film is much faster than the rest of the drop; after a short transient, the latter spreads over the electrowetting film by draining into it. By employing matched asymptotics, we are able to elucidate this unique mechanism, justified by the reasonable agreement with numerical and experimental results. Unlike the usual electrowetting-on-dielectric configuration where the field singularity at the contact line produces a static change in the contact angle consistent with the Lippmann equation, we show that the parallel electrode configuration produces a bulk negative Maxwell pressure within the drop. This Maxwell pressure increases in magnitude toward the contact line due to field confinement and is responsible for a bulk pressure gradient that gives rise to a front-running spontaneous electrowetting film.

Journal Article↗
PubMed2006

Breath figure templated self-assembly of porous diblock copolymer films.

Porous polyethylene oxide-b-polyfluorooctylmethacrylate (PEO-b-PFOMA) diblock copolymer films were drop cast onto substrates from Freon (1,1,2-trichlorotrifluoroethane) in a humid atmosphere. The pores in the films exhibit long range hexagonal order in some cases, depending on the PFOMA-to-PEO molecular weight ratio. Films with the best ordered pores were formed with PFOMA-to-PEO ratios of 70 kDa:2 kDa. The pores in the polymer films derive from water droplets that condense as Freon evaporates. The polymer stabilizes the water droplets, or "breath figures," which act as an immiscible template that molds the porous film. Increased polymer hydrophobicity reduces the water wettability of the air/Freon interface, which in turn decreases water droplet nucleation, thus influencing the final pore size and spatial order in the polymer films. We describe how water droplet nucleation influences the final pore size and packing order in the polymer films.

Journal Article↗
PubMed2000

Chain segment order in polymer thin films on a nonadsorbing surface: A NMR study

The dynamics of polydimethylsiloxane chains confined between air and a solid, wettable, and nonadsorbing surface is investigated using deuterium NMR. Chain segments in such molten films undergo uniaxial reorientations around the normal to the surfaces. The influence of the film thickness proves this dynamic uniaxiality involves rapid segment diffusion between both surfaces which impose a high degree of planar orientation. This segmental ordering is observed over a thickness range which depends on the degree of chain entanglements.

Journal Article↗
PubMed2001

Rate-dependent slip of Newtonian liquid at smooth surfaces.

Newtonian fluids were placed between molecularly smooth surfaces whose spacing was vibrated at spacings where the fluid responded as a continuum. Hydrodynamic forces agreed with predictions from the no-slip boundary condition only provided that flow rate (peak velocity normalized by spacing) was low, but implied partial slip when it exceeded a critical level, different in different systems, correlated with contact angle (surface wettability). With increasing flow rate and partially wetted surfaces, hydrodynamic forces became up to 2-4 orders of magnitude less than expected by assuming the no-slip boundary condition that is commonly stated in textbooks.

Journal Article↗
PubMed2002

Kinetics of capillary condensation in nanoscopic sliding friction.

The velocity and humidity dependence of nanoscopic sliding friction has been studied on CrN and diamondlike carbon surfaces with an atomic force microscope. The surface wettability is found to be decisive. Partially hydrophilic surfaces show a logarithmic decrease of friction with increasing velocity, the slope of which varies drastically with humidity, whereas on partially hydrophobic surfaces we confirm the formerly reported logarithmic increase. A model for the thermally activated nucleation of water bridges between tip and sample asperities fully reproduces the experimental data.

Journal Article↗
PubMed2003

Kinetic roughening in two-phase fluid flow through a random Hele-Shaw cell.

A nonlocal interface equation is derived for two-phase fluid flow, with arbitrary wettability and viscosity contrast, c=(mu(1)-mu(2))/(mu(1)+mu(2)), in a model porous medium defined as a Hele-Shaw cell with random gap b(0)+delta b. Fluctuations of both capillary and viscous pressure are explicitly related to the microscopic quenched disorder, yielding conserved, nonconserved, and power-law correlated noise terms. Two length scales are identified that control the possible scaling regimes and which scale with capillary number Ca as l(1) approximately b(0)(cCa)(-1/2) and l(2) approximately b(0)Ca-1. Exponents for forced fluid invasion are obtained from numerical simulation and compared with recent experiments.

Journal Article↗
PubMed2003

Capillary force on a nanoscale tip in dip-pen nanolithography.

Monte Carlo simulation has been used to characterize the capillary force due to the condensation of a liquid meniscus between a tip with a nanoscale asperity and a flat surface. To consider both hydrophobic and hydrophilic molecules coating the tip as a model of dip-pen nanolithography, tips with various wettabilities are studied. The capillary force due to the meniscus is calculated for various saturations (humidities). We have implemented a thermodynamic integration technique that can project the force into energetic and entropic contributions. In most cases, the force is mainly energetic in origin. At the snap-off separation where the meniscus disappears, the tip feels a significant entropic force at high saturation. Our calculation shows nonmonotonic behavior of the pull-off force as a function of saturation, which is in qualitative accord with experiments.

Journal Article↗
PubMed2004

Dipole-dependent slip of Newtonian liquids at smooth solid hydrophobic surfaces.

We present atomic force microscopy observations of the "effective" slippage of various nonpolar and polar liquids on alkylsilane coated glass surfaces. For small contact angle nonpolar liquids, the slip length decreases as one approaches a wetting transition. However, for large contact angle polar liquids it is found that the slip length is primarily influenced by the dipole moment, rather than the wettability of the liquid for the surface, where the slip length decreases with increasing dipole moment.

Journal Article↗
PubMed2004

Dynamical model for chemically driven running droplets.

We propose coupled evolution equations for the thickness of a liquid film and the density of an adsorbate layer on a partially wetting solid substrate. Therein, running droplets are studied assuming a chemical reaction underneath the droplets that induces a wettability gradient on the substrate and provides the driving force for droplet motion. Two different regimes for moving droplets--reaction-limited and saturated regime--are described. They correspond to increasing and decreasing velocities with increasing reaction rates and droplet sizes, respectively. The existence of the two regimes offers a natural explanation of prior experimental observations.

Diffusion↗
PubMed1993

The interplay of light and the circadian clock. Independent dual regulation of clock-controlled gene ccg-2(eas).

Ambient light is the major agent mediating entrainment of circadian rhythms and is also a major factor influencing development and morphogenesis. We show that in Neurospora crassa the expression of clock-controlled gene 2 (ccg-2), a gene under the control of the circadian clock and allelic to the developmental gene easy wettable (eas), is regulated by light in wild-type strains. Light elicits a direct and important physiological effect on ccg-2(eas) expression as demonstrated using several mutant Neurospora strains. In white collar mutants (wc-1 and wc-2) that are "blind" to blue light, ccg-2(eas) mRNA shows no variation following illumination with saturating light. By contrast, ccg-2(eas) mRNA is photoinduced in clock-null strains such as frequency (bd;frq). The results in the clock mutants show that an intact circadian oscillator is not required for light induction of ccg-2(eas). Thus, ccg-2(eas) is subject to a dual regulation that involves separable regulation by light and circadian rhythm.

Alleles↗
PubMed1993

Identification and characterization of MPG1, a gene involved in pathogenicity from the rice blast fungus Magnaporthe grisea.

Differential cDNA cloning was used to identify genes expressed during infectious growth of the fungal pathogen Magnaporthe grisea in its host, the rice plant. We characterized one of these genes, MPG1, in detail. Using a novel assay to determine the proportion of fungal biomass present in the plant, we determined that the MPG1 transcript was 60-fold more abundant during growth in the plant than in culture. Mpg1 mutants have a reduced ability to cause disease symptoms that appears to result from an impaired ability to undergo appressorium formation. MPG1 mRNA was highly abundant very early in plant infection concomitant with appressorium formation and was also abundant at the time of symptom development. The MPG1 mRNA was also expressed during conidiation and in mycelial cultures starved for nitrogen or carbon. MPG1 potentially encodes a small, secreted, cysteine-rich, moderately hydrophobic protein with the characteristics of a fungal hydrophobin. Consistent with the role of the MPG1 gene product as a hydrophobin, Mpg1 mutants show an "easily wettable" phenotype. Our results suggest that hydrophobins may have a role in the elaboration of infective structures by fungi and may fulfill other functions in fungal phytopathogenesis.

Amino Acid Sequence↗
PubMed1990

Use of an electrostatic sprayer for control of anopheline mosquitoes.

The Electrodyn sprayer was compared with a compression sprayer (Hudson X-pert) for residual application of cypermethrin, a pyrethroid insecticide, to control the malaria vectors Anopheles arabiensis Patton and An. funestus Giles in experimental huts at Magugu in Tanzania. The time taken for hut spraying, 2-2.5 min per hut, was similar for both types of sprayer. Two or three huts were treated internally with cypermethrin at 40 or 80 mg a.i./m2 using Electrodyn formulation for comparison with 80 mg a.i./m2 using wettable powder formulation. Each of the twelve huts (including five untreated controls) was fitted with window exit traps and either louvre or verandah traps for mosquito sampling. The Electrodyn sprayer was fitted with a pair of elbowed deflectrodes to direct the positively charged spray droplets onto walls and ceiling. All treatments gave 94-100% mortality-rates of indoor-resting anophelines throughout the evaluation period of 11 weeks post-spray. Reductions of An.arabiensis and An.funestus females by 10-42% and 62-91%, respectively, in rooms and by 72% and 51% in exit traps indicated that cypermethrin deterred mosquitoes from entering the huts. Overall mortality-rates of mosquitoes were 66% of both species in huts treated with 40 mg/m2 Electrodyn, 43% An.funestus and 71% An.arabiensis due to 80 mg/m2 Electrodyn formulation and 49% An.funestus and 64% An.arabiensis due to 80 mg/m2 WP formulation (no significant differences). It is concluded that the Electrodyn sprayer with deflectrodes is a convenient and effective means of residual house-spraying with pyrethroid insecticide for malaria vector control.

Aerosols↗
PubMed1991

Control of tsetse and trypanosomiasis transmission in Uganda by applications of lambda-cyhalothrin.

The pyrethroid insecticide lambda-cyhalothrin was evaluated in field trials against Glossina f.fuscipes and sleeping sickness transmission in Iyolwa sub-county, Tororo District, Uganda. The insecticide was applied selectively to the resting-sites of tsetse, by bush-spraying, using 10% wettable powder (10WP) formulation at an application rate of 11.6 g a.i./ha over an area of 28 km2, or by a 2% Electrodyn formulation (2ED) applied at 0.9 g a.i./ha over 30 km2. In a third trial area of 32 km2, 215 pyramidal traps treated with lambda-cyhalothrin 100 mg/m2 were set. The best impact was obtained with 10WP lambda-cyhalothrin which eliminated tsetse within 1-2 months, whereas G.f.fuscipes persisted at very low density in part of the area treated with 2ED lambda-cyhalothrin. In both treated areas the numbers of human sleeping sickness cases fell to no more than one per month, compared with four to twelve per month previously. The overall rate of cattle trypanosomiasis (T.brucei and T.vivax) was also reduced slightly. Insecticide-treated traps remained fully effective for at least 6 months under field conditions and catches were reduced 20-90-fold. These results in the control of tsetse and trypanosomiasis transmission lead us to recommend lambda-cyhalothrin for tsetse control operations.

Aerosols↗
PubMed1991

Comparative use of bendiocarb and DDT to control Anopheles pseudopunctipennis in a malarious area of Mexico.

The state of Sinaloa has one of the highest and most persistent malaria transmission levels in Mexico. Due to this situation, with resistance of the vector Anopheles pseudopunctipennis Theobald to DDT, the carbamate insecticide bendiocarb was evaluated as an alternative to DDT for residual house-spraying in village-scale trials during 1985-87. Application rates of the active ingredient per square metre of sprayable surface (ai/m2) were 0.4 g bendiocarb 80% wettable powder (80WP) and 2 g DDT 75% WP. Both insecticides failed to control mosquito populations. Human-bait mosquito densities were not altered as a result of insecticide spraying and human-bait collected mosquito mortality rates were low, suggesting little pre-biting insecticide contact due to avoidance or insufficient resting time indoors. Lower densities of indoor-resting mosquitoes were observed with DDT as opposed to bendiocarb treated houses. Anopheline mortality was higher (98-100%) when exposed for 1 h to 1% bendiocarb in standard WHO susceptibility tests and wall bioassays. Mortality-rates of 15-48% due to 1 h exposure to 4% DDT indicated that this insecticide may continue to be partially effective. House curtain and mark-recapture mosquito studies indicated that DDT produced higher excito-repellency than bendiocarb, as reflected by more mosquito landings but lower feeding rates, shorter resting period and earlier exit time from DDT sprayed houses. In the absence of insecticide, more than 50% of blood-fed An.pseudopunctipennis females exited from houses within 2-4 h of release, showing exophilic behaviour. The outdoor/indoor density ratio indicated that the majority were exophagic. These behavioural characteristics limit the usefulness of any residual insecticide against An.pseudopunctipennis.

Aerosols↗