Difference In Surface Tension Research Articles

Simulation of binary dispersion system of droplets with size and surface tension difference under Couette flow

Simulation of binary dispersion system of droplets with size and surface tension difference under Couette flow

Motion with Memory of a Self-Propelled Object

The concept of self-propelled objects is important for the understanding of biological mobility, as well as for the development of autonomous devices in medicine and engineering. In this study, a simple self-propelled object, driven by a difference in surface tension, was found to exhibit intermittent self-motion (alternately in motion and at rest) in an annular water channel, with resting positions and features of motion in subsequent cycles remaining almost the same as those previously visited; that is, memories of the resting positions and features of motion were observed. The occurrence of the memory phenomenon was found to depend on the relationship between the resting time and the period for one lap of the annular channel. The mechanism of memory is discussed in terms of the distribution of surface-active molecules and local surface tension at the resting positions.

Heat Transfer in Microchannels—2012 Status and Research Needs

Abstract Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Droplet-induced abnormal bending of micro-beams

We present a theoretical study of the abnormal deformation of micro-beams induced by a droplet placed on their top/bottom surfaces. The bending moments and deflections of the micro-beams are derived in consideration of the surface tension and Laplacian pressure difference of the droplet. The deflection curves show that the cantilever inevitably tilts upwards with a droplet on its top surface, and most parts of the simply supported beam and clamped–clamped beam will bend upwards when a droplet is adhered to their bottom surfaces. These anomalous deformation behaviors have been verified by our finite element simulations. We also demonstrate what extent of the boundary conditions will affect the deflection curves of the micro-beams during the variation of the droplet’s parameters. Understanding this abnormal bending mechanism of the beams can be beneficial to some industry applications, such as the micro-electro-mechanical systems (MEMS) and the micro/nano-measurement, which can provide the inspiration of engineering more precision micro-sensors.

Thermocapillary effects in foams: Thermal diffusion through a bubble staircase and impact on shear modulus

A temperature gradient is applied to a single layer of bubbles in a regular two-dimensional foam (a ‘bubble staircase’), allowing a thermocapillary effect to occur and alter the structure and, thus, the shear modulus. In this model, local surface tension is a function of local surfactant concentration and local temperature. As thermal energy diffuses through the structure from an elevated (or lowered) temperature at a wall, the surface tension changes; in turn, surface tensions inequities result in a Marangoni flux of surfactant along the interface, re-adjusting surface tension inequities. An additional physicochemical equilibration of the film with a nearby bulk reservoir of surfactant (e.g., liquid-containing Plateau borders) allows for interaction of the films with their environment. As the temperature at the wall is increased, surface tension difference between the liquid on the hotter side and that on the cooler side also increases, causing migration of the vertices separating films towards the cooler side. This change in structure affects also the elastic modulus of the foam. In both simulation results and theoretical predictions, temperature variations on the order of tens of degrees are significant.

Parenchymal mechanics, gas mixing, and the slope of phase III

A model of parenchymal mechanics is revisited with the objective of investigating the differences in parenchymal microstructure that underlie the differences in regional compliance that are inferred from gas-mixing studies. The stiffness of the elastic line elements that lie along the free edges of alveoli and form the boundary of the lumen of the alveolar duct is the dominant determinant of parenchymal compliance. Differences in alveolar size cause parallel shifts of the pressure-volume curve, but have little effect on compliance. However, alveolar size also affects the relation between surface tension and pressure during the breathing cycle. Thus regional differences in alveolar size generate regional differences in surface tension, and these drive Marangoni surface flows that equilibrate surface tension between neighboring acini. Surface tension relaxation introduces phase differences in regional volume oscillations and a dependence of expired gas concentration on expired volume. A particular example of different parenchymal properties in two neighboring acini is described, and gas exchange in this model is calculated. The efficiency of mixing and slope of phase III for the model agree well with published data. This model constitutes a new hypothesis concerning the origin of phase III.

Capillary buckling of a floating annulus

We describe the out-of-plane buckling of a flexible annulus floating on a bath of water as surfactant molecules are added outside the annulus. The difference in surface tension induces compressive stresses, which result in regular orthoradial wrinkles beyond a critical difference in surface tension. The wrinkles first appear in the vicinity of the inner edge of the annulus and progressively grow as the concentration of surfactant is increased. Conversely, the wavenumber remains constant and relies on a simple balance between gravity and the bending stiffness of the membrane within the range of our experimental parameters. Our experiments fall outside the regimes explored in the literature for similar situations, and we propose an approximate analytical description that completes the existing theoretical grounds.

An Onset of Micromixing Flow Utilizing Marangoni Force

Many kinds of micromixer have been proposed and studied in this decade. The mixing mechanism of most of them has been mainly dependent on molecular diffusion. Therefore, adopting small diffusive distance has been primary measure of mixing enhancement. In this study, we have investigated a novel mixing process with two liquids having difference in surface tension, by the onset of Marangoni effect. Marangoni effect is typical phenomenon on gas-liquid free interface when there is the variation in surface tension. We conducted basic experiments in a microchannel using an air-bubble surface as the gas-liquid interface for the onset of Marangoni covection. We also performed CFD simulation to compare with the experiments. In the experiments, we successfully observed vortex flow which can contribute to species mixing, on the bubble in a few seconds after the moment of contact of the two liquids. Moreover, the CFD simulation could reproduce the vortex flow successfully.

5PM3-PMN-045 表面張力駆動型マニピュレータの開発(微細加工とMEMS,ポスターセッション)

We propose and demonstrate the autonomous tweezers driven by surface tension without any actuators. The tweezers are composed of only a pair of the polydimthylsiloxane (PDMS) cantilevers having different wettability. Since the front and back surfaces of the cantilevers have different wettability, they can bend due to the difference of surface tension and hydrostatic pressure exerted on each surface when they come in contact with the surface of liquid. Using this self-bending phenomenon, we succeed in not only grasping but also releasing a floating object under proper conditions. The autonomous tweezers will be useful for damage-less manipulation of biological samples without any actuators and external energy source.

Peptide-powered boats

The release and self-assembly of peptides from metal–organic frameworks creates surface tension differences that can fuel the cruising motion of the framework, and a microscale 'boat' wrapped around a framework particle, at the air/liquid interface.

Phase separation and structure evolution of ternary Al–Cu–Sn immiscible alloy under ultrasonic levitation condition

The containerless rapid solidification of liquid Al–15%Cu–55%Sn immiscible alloy is accomplished by single-axis ultrasonic levitation. A maximum undercooling of 209K (0.23TL) is obtained for the alloy melt at a cooling rate of about 100K/s. The phase separation pattern transforms from the vertical-type macrosegregation under normal condition into the lateral-type macrosegregation under ultrasonic levitation. The enhancement of both undercooling and cooling rate induces the refinement of microstructure and the remarkable extension of solute solubility in (Sn) phase. Theoretical analyses show that the morphology transition mainly depends on the density and surface tension differences between Al-rich and Sn-rich liquid phases. The centrifugal migration of Sn-rich droplets caused by sample rotation is the major dynamic mechanism responsible for the macrosegregation evolution under ultrasonic levitation condition.

Unconventional Multiple Ring Structure Formation from Evaporation-Induced Self-Assembly of Polymers

The formation of multiring deposits of poly(2-vinylpyridine) (P2VP) from the evaporation of a P2VP-(2,6-lutidine + water) drop on a glass substrate does not conform to the conventional pinning-depinning mechanism. Instead, ringlike deposits are formed when the droplet undergoes several cycles of spreading and receding where, for each spreading event, a P2VP ridge is formed at the contact line when the polymer flows toward the outward advancing edge. The complex interplay between an outward solutal-Marangoni flow due to a higher concentration of the polymer at the contact line and an inward solvent-Marangoni flow arising from the differences in volatilities and surface tensions of the pure solvent components plays an important role in enhancing the droplet spreading rate. The newly discovered surface patterning mechanism has important implications in the development of novel techniques for inducing self-assembly of functional materials from evaporating drops.

On the Role of the Difference in Surface Tensions Involved in the Allosteric Regulation of NHE-1 Induced by Low to Mild Osmotic Pressure, Membrane Tension and Lipid Asymmetry

The sodium-proton exchanger 1 (NHE-1) is a membrane transporter that exchanges Na+ for H+ ion across the membrane of eukaryotic cells. It is cooperatively activated by intracellular protons, and this allosteric regulation is modulated by the biophysical properties of the plasma membrane and related lipid environment. Consequently, NHE-1 is a mechanosensitive transporter that responds to osmotic pressure, and changes in membrane composition. The purpose of this study was to develop the relationship between membrane surface tension, and the allosteric balance of a mechanosensitive transporter such as NHE-1. In eukaryotes, the asymmetric composition of membrane leaflets results in a difference in surface tensions that is involved in the creation of a reservoir of intracellular vesicles and membrane buds contributing to buffer mechanical constraints. Therefore, we took this phenomenon into account in this study and developed a set of relations between the mean surface tension, membrane asymmetry, fluid phase endocytosis and the allosteric equilibrium constant of the transporter. We then used the experimental data published on the effects of osmotic pressure and membrane modification on the NHE-1 allosteric constant to fit these equations. We show here that NHE-1 mechanosensitivity is more based on its high sensitivity towards the asymmetry between the bilayer leaflets compared to mean global membrane tension. This compliance to membrane asymmetry is physiologically relevant as with their slower transport rates than ion channels, transporters cannot respond as high pressure-high conductance fast-gating emergency valves.

Water Activity

Microorganisms require water for their metabolic activities. Only a fraction of water in foodstuffs, the so-called free water, is available for this purpose. The amounts of free water previously estimated by two different methods (Frosch et al. (2010), Frosch et al. (2011), and Low (1969)) are compared for aqueous solutions of four electrolytes, NaCl, NH4Cl, Na2SO4, (NH4)2SO4: (i) vapour pressure measurements of the solutions relative to that of pure water (water activities) and (ii) low-wavenumber Raman spectra in the R(ν)-representation. For each electrolyte deviations were found between results from the two methods. All water molecules in the illuminated volume contribute to the Raman data. The vapor pressure measurements refer to water molecules at the water/atmosphere interface where surface tension is important. Differences in surface tension for the four electrolytes qualitatively explain deviations between the amounts of “free water” observed by the two methods.

表面張力、界面張力差を考慮した異種液滴間衝突のSPHシミュレーション

In this study, SPH (Smoothed Particle Hydrodynamics) simulations of binary droplets collision in free space were carried out including effects of surface tension formulated based on Cahn-Hilliard free energy theory, and the effects of the differences in density and surface tension of fluids on the collision behavior were investigated. The spatial gradient appropriate for dealing with particles with different densities was derived by using modified discrete equations. The simulation result of head-on collision between identical micrometer-scale droplets corresponded well to the experimental result. Upon collision of droplets with different surface tension coefficients, the liquid-liquid interface line at the free surface moved toward the liquid with higher surface tension coefficient immediately after the contact due to the Marangoni effect, and then the liquid with lower surface tension coefficient was covered with that with higher one.

Interplay of electrohydrodynamic structure formation and microphase alignment in lamellar block copolymers

Electrohydrodynamic (EHD) destabilisation of a lamellar-forming block-copolymer (BCP) film gives rise to hierarchical pattern formation on the micrometre and 10 nm length scale in one single step. Two BCP morphologies were found in the columns that formed from a homogeneous surface instability. At low electric field strength, coaxial concentric lamellae indicate the dominance of surface tension differences of the BCP components over electric field effects. Outward opening lamellae are caused by lateral electric field gradients at sufficiently high field strength. Nucleated instabilities at low field strength give rise to stable bulls-eye-type ring structures consisting of BCP lamellae aligned along the ring perimeter. This is explained in terms of a free energy argument, illustrating the role of the internal energy of the BCP stack, resisting the deformation of the ring. Finally, the EHD experimental configuration was used to fabricate large monodomains of standing oriented lamellae.

Wettability Conversion from Superoleophobic to Superhydrophilic on Titania/Single-Walled Carbon Nanotube Composite Coatings

Superoleophobic surfaces were demonstrated on perfluorosilane-rendered titania (TiO(2))/single-walled carbon nanotube (SWNT) composite coatings. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that SWNTs play a key role in the formation of overhanging structures and the nanoscale roughness on the coating surface, which compose the two critical morphologic factors for a superoleophobic surface. The wettability conversion from superoleophobic to superhydrophilic of the composite coatings was realized by the gradual decomposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) on the coating surface using UV irradiation. Contact angle measurement on both smooth TiO(2) surface and rough composite coating surface under different UV irradiation time revealed that the wetting behavior of the liquids on the composite coating surface passes from the Cassie to the Wenzel and finally to the inversed-Cassie regime. Different liquids show different irradiation time for the wetting state change. By controlling the UV irradiation dose, liquids with surface tension difference smaller than 5 mN/m can exist in completely converse wetting states on the same coating surface, that is, superphobic for one liquid while superphilic for another with lower surface tension. Mixed organic liquids with different surface tension can be completely separated through a coated grid using this wettability tuning technique.

Modeling rhamnolipids production by Pseudomonas aeruginosa from immiscible carbon source in a batch system

A mathematical model to predict the rhamnolipids production by Pseudomonas aeruginosa from oleic acid in a two phase liquid-liquid batch reaction system, was developed in this study. The model was based on two theoretical assumptions: 1) the convective oleic acid mass transfer is coupled to a bioreaction in the aqueous liquid bulk, and 2) the volume of the immiscible oleic acid drops and the saturation concentration at the interface are a function of rhamnolipids production. The model was able to accurately predict the experimental growth of the Pseudomonas aeruginosa strain, and the rhamnolipids production data with oleic acid as carbon source. This mathematical approach indicated a high correspondence between the saturation dimensionless profiles of oleic acid at the interface and the experimental profiles of surface tension difference. This modeling approach may constitute a useful tool in the design and scaling-up of bioreactors applied to the production of biosurfactants with immiscible carbon sources.

CFD simulation of the two-phase flow for a falling film microreactor

Falling film microreactors, which provide very high specific interfacial area, have become a promising solution to the fast and strongly exothermic/endothermic gas–liquid reaction systems. A computational fluid dynamic simulation of the two-phase flow for a falling film microreactor is presented using the volume of fluid (VOF) model. The hydrodynamic characteristics, from both 2-D and 3-D simulations, including liquid film thickness, velocity, pressure and shear stress profiles, are analyzed. 2-D simulation is adopted for the study of the relationship of liquid flow rate and film thickness, as well as the effects of gas flow rate, surface tension, liquid viscosity and pressure difference on the liquid flow rate. 3-D simulation is necessary to provide the comprehensive flow profiles. Although the system is in the laminar flow regime, the liquid film features a wavy structure and the velocity profiles are complex.

Improving the wettability of aluminum on carbon nanotubes

The wetting of a metal on carbon nanotubes is fundamentally difficult due to the unusually large difference between their surface tensions and is a bottleneck for making metal–carbon nanotube (CNT) composites. Here, we report a simple method to enhance the wettability of metal particles on the CNT surface by applying aluminum, which is the material with the largest surface tension. This method involves two steps: (i) Al nanoparticles are decorated on multiwalled carbon nanotubes by electroplating and (ii) Al powder is further spread on Al-electroplated CNTs, followed by high-temperature annealing to accommodate complete wetting of the aluminum. The large surface tension difference is overcome by forming strong Al C covalent bonds initiated by defects of the CNTs. The decrease in the D-band intensity, the G-band shift in the Raman spectroscopy and the formation of Al C covalent bonds, as confirmed by X-ray photoelectron spectroscopy, were in agreement with our structural model of CNT vacancy O Al determined by density functional calculations.