Whispering gallery mode resonance excitations on a partially gold coated bottle microresonator

The study on the excitations of whispering gallery mode (WGM) resonances optical bottle microresonator (BMR) partially coated with a thin gold Au metal film is presented. The BMR was fabricated through “soften-and-compress” technique on a small section of standard optical fibre. Depositing Au particles on the spheroidal curvature of the BMR surface yields a thin metal-film of a meniscus profile with 200 nm maximum gold thickness and tapered edges. A polarization resolved experimental setup was used to excite TE- and TM-mode resonances. Coupling strengths of the excited WGMs would vary with different coupling arrangements relative to the position of the meniscus Au film. Calculated Q-factor values of composite TE and TM mode resonances were determined to be in the range 1800 and 2700, respectively.

Calculations of the Meniscus Force and the Contact Force Formed in the Microcontacts of a Rough Surface and a Smooth, Rigid Surface with a Thin Water Film

ABSTRACTA present model is developed to calculate the adhesion meniscus force due to a rough surface with surface asperity in contact with a smooth, rigid flat covered by a thin water film. The original thickness of this film before surface contacts is dependent upon the relative humidity in the air. Microcontact deformations of surface asperities in the elastic, elastoplastic, and fully plastic regimes are included in the present model under a normal load. The new water film thickness under the condition of microcontact deformations is considered changing with the normal load, and it is obtained from the equation developed on the basis of the volume conservation principle for the new film thickness and the water film volume displaced by the asperities heights dipping in the film. The meniscus profile is also calculated from the balance of the surface tension force and the pressure difference force across the meniscus profile if the new film thickness is available. Water film thickness and the meniscus force are increased by decreasing the mean separation of two contact surfaces, or increasing the relative humidity, or increasing the plastic index. A significant difference in the meniscus force is found between the present model and the model of the literature, which is enhanced by either decreasing the mean separation, or raising the plasticity index, or increasing the relative humidity. The effects of the meniscus force on the load capacity are also evaluated at different mean separations, relative humidity and plasticity indices.

Numerical Study of an Evaporating Meniscus on a Moving Heated Surface

The present study is performed to numerically analyze an evaporating meniscus bounded between the advancing and receding interfaces on a moving heated surface. The numerical scheme developed for analyzing interface motion during bubble growth in pool boiling has been applied. A column of liquid is placed between a nozzle outlet and a moving wall, and calculations are done in two dimensions with a fixed distance between the nozzle and the wall. The results show that the wall velocity creates a circulation near the meniscus base, resulting in transient heat conduction. The local wall heat transfer is found to vary significantly along the meniscus base, the highest being near the advancing contact line. The heat transfer coefficient is found to depend on the advancing contact angle and wall velocity but is independent of the wall superheat. Reasonable agreement is observed when the meniscus profile and heat transfer results obtained from the numerical simulation are compared to the experimental data.

Computational Modeling of Macroscopic Transportation in Continuous Castings

Macroscopic transportation in a continuous casting is modeled using the SIMPLER algorithm and the finite difference method. The simulation software Visual Cast has been developed to describe meniscus profile, fluid flow, solidification and centreline segregation based on the volume of fluid function and the mass, momentum, energy, and species conservation equations. It is found that the meniscus profile is coherently related to the position of circumfluence in the continuous casting mold. When the vortex above the flow jet from the nozzle is close to the meniscus, the level fluctuation becomes prominent. The centreline segregation is affected by the thermosolutal convection. The species concentration in the mush of the strand rises quickly and reaches a maximum at the end of the solidification.

Characteristics of an Evaporating Thin Film in a Microchannel

The thin-film region of an evaporating meniscus is investigated through an augmented Young-Laplace model and the kinetic theory-based expression for mass transport across a liquid-vapor interface. A fourth-order differential equation for the thickness profile is developed and the boundary conditions at the beginning of the thin-film region are discussed in detail. A perturbation on the initial thickness is employed to avoid the evaporation being totally suppressed all along the meniscus. The role of capillary pressure in controlling the meniscus profile and rate of liquid supply is detailed. The evaporation heat transfer coefficient is greatly suppressed at the beginning of the thin-film region due to disjoining pressure; in the intrinsic meniscus, evaporation is suppressed due to capillary pressure, especially for low wall superheat. The importance of the thin-film region in determining the overall heat transfer is shown to depend on the channel size and degree of superheat.

Meniscus Forces and Profiles: Theory and Its Applications to Liquid-Mediated Interfaces

A theory for obtaining meniscus forces and profiles for any given liquid-mediated interface is presented that includes the effects of surface interactions, adsorption and evaporation of liquid films. The meniscus force is obtained from the derivative of the total free energy of liquid-mediated interface, which requires the meniscus profile to be known. The meniscus profile is the solution of a second-order differential equation, as derived from Pascal’s law for static incompressible liquids with inclusion of surface interactions. For nonvolatile liquid films, the total liquid amount at the interface is a conserved quantity, whereas for volatile liquids, the liquid films are in thermodynamic equilibrium with their respective vapor phase. Two typical types of initial liquid conditions are considered. Type I represents the case in which one surface is wet and the other is initially dry, having a finite contact angle with the liquid. Type II represents the situation in which both surfaces are wet by either a liquid or by two different liquids before making contact. If two or more types of liquids are involved at the interface, miscibility of the liquids and interactions due to other liquid(s) have to be also considered. For contacts with azimuthal geometry, which is merely a mathematical convenience, such as ellipsoidal/spherical, conical or crater, the theory generates several analytical formulae for calculating meniscus forces without involving meniscus profiles. These formulae can be handily applied to various surface probes techniques such as Scanning Probe Microscopy or Surface Force Apparatus. The proposed theory is also applicable to “meniscus rings” formed around crater geometry, such as encountered in laser-textured magnetic disks. In this case, the outer meniscus ring can be asymmetric to the inner meniscus ring if no liquid passage exists between the inner and outer meniscus ring. Even for the case of spherical contact geometry, the calculated meniscus profile is very nonspherical with a much larger volume than that of the widely assumed spherical meniscus profile for Type I conditions, leading to an under-estimation of the meniscus force in the previous models. It is found that for a spherical or a crater contact geometry, the surface interactions have little effect on the meniscus force provided the lateral meniscus dimension is much smaller than the radius of the sphere or of the crater. However the surface interactions have a large effect on the meniscus force for other contact geometries, such as conical contact geometry. The calculated meniscus forces are compared with the normal component of the stiction force measured at the laser textured surfaces and good agreement is found. The calculated meniscus profiles are also found in good agreement with that measured using light interferometer technique between two cross cylinders. One very interesting finding of our theory is that the meniscus volume grows first and may then shrink, as observed experimentally by others, because the initially dry surface become wetted and the boundary conditions change over from Type I to Type II.