G. Paul Neitzel

Optical levitation and transport of microdroplets: Proof of concept

A new technique for microfluidic transport of liquid droplets in “lab-on-a-chip” (LOC) applications is described in which droplets are levitated above or between solid planar surfaces through the use of thermocapillarity, the variation in a liquid’s surface tension with temperature. Levitated liquid droplets are not in contact with solid surfaces and so may be transported from point-to-point along arbitrary paths with little friction. In addition, the lack of liquid-solid contact virtually eliminates the potential for sample-to-sample contamination. The new technique therefore addresses three issues associated with existing LOC devices employing microchannels: droplet pathway constraints, transport speed, and sample-to-sample contamination.

Lubrication analysis of thermocapillary-induced nonwetting

Recent interest in the phenomenon of thermocapillary-induced noncoalescence and nonwetting has produced experimental evidence of the existence of a film of lubricating gas that prevents the two surfaces in question (liquid–liquid for noncoalescence; liquid–solid for nonwetting) from coming into contact with one another. Measurements further indicate that the pressure distribution in this film creates a dimpled liquid free-surface. Lubrication theory is employed to investigate the coupled effects of liquid and gas flows for a two-dimensional nonwetting case of a hot droplet pressed toward a cold wall. The analysis focuses on the respective roles of viscous and inertial forces on droplet deformation. Resultant droplet shapes show an influence of gas viscosity maintaining nonwetting and of inertia contributing to a dimple. Previous analyses of thermocapillary-driven flow in liquid layers and droplets model the gas as purely passive which cannot be the case in the present application.

Failure of thermocapillary-driven permanent nonwetting droplets

A droplet may be prevented from molecular contact with a solid surface by providing a thin, lubricating film of surrounding fluid between the solid and liquid surfaces. In this study, we exploit thermocapillary convection, caused by a temperature difference maintained between the droplet and the unwetted surface, to provide this lubricating film. This state may be sustained indefinitely (permanent nonwetting) if the load applied to the droplet does not exceed a threshold. Failure of such systems may be categorized as either film or pinning failures, depending on whether the lubrication film is breached, resulting in a molecular contact between the droplet and the solid surface, or the droplet is forced from its support by losing its pinning contact line. In this work, loads that trigger film and pinning failures are quantified, and their mechanisms explained. Results show that larger loads can be sustained for systems with an elevated temperature difference and for droplets of higher viscosity.

Multiscale modelling in the numerical computation of isothermal non-wetting

A state of permanent, isothermal non-wetting of a solid surface by a normally wetting liquid may be achieved if the surface moves tangentially to a liquid drop that is pressed against it. Surrounding gas is swept into the space between the liquid and solid creating a lubricating film that prevents wetting. The length scales of the drop and the film are typically three or more orders of magnitude different, making numerical simulation difficult from a resolution standpoint. The present paper focuses on a hybrid approach employing lubrication theory for the thinnest portions of the gas film and numerical simulation for the liquid and outer gas phases.

Thermal instability with radiation by the method of energy

Abstract Energy-stability theory is applied to the case of radiation heat transfer in an optically thin, quiescent fluid layer heated from below and bounded by rigid, black, perfectly conducting planes. The radiation term in the energy equation destroys the quadratic character of the energy identity. It is shown, however, that the right-hand side of the energy identity can be bounded by a suitable quadratic term for all physically allowable disturbances. The result is a conditional stability limit dependent upon disturbance amplitude. Results are computed for a variety of cases; these are compared to existing linear and energy stability results.

Stephen H. Davis – 70, and counting

Celebrating the occasion of his 70th birthday, observations are offered on the life and technical contributions of Stephen H. Davis, Editor of the Journal of Fluid Mechanics .

Control-Volume Momentum Flux: Getting the Sign Correct

When describing the application of control-volume analysis to linear-momentum-balance problems, students are cautioned that there are two different things to take into consideration when determining the proper sign for the momentum-flux term. This decision can be simplified through reliance on the choice of the coordinate system and control surface to determine the sign.