Glovebox Studies
The IFFD (Internal Flows in Free Drops) investigation were carried out on STS-95 produced good video images with optimal resolution of the internal tracer particles will
allow the accurate measurement of the internal motion of the liquid. The first demonstration of non-contact fissioning of a single drop into two parts was obtained with a static sound
field. In addition, the new technique for the accurate, acoustically-assisted drop deployment in microgravity has also been verified together with the feasibility of quiescently positioning a
partially wetted drop at the end a sting. Thermocapillary flows were observed with a free drop in microgravity, and clear evidence of an increase in the internal circulation in the drop was detected as
the sting heater was activated in the vicinity of the drop. Both contacting and non-contacting heater configurations were studied. A much higher rate of successful drop deployment has been obtained in STS-95 when compared to STS-94. This has been attributed to the elimination of the protective cover around the ultrasonic levitator.
Ground-Based Experiments
We have continued ground-based measurement of the thermocapillary flows in electrostatically levitated and laser-heated drops. Convincing evidence is being gathered that
these thermocapillary-driven flows quickly couple to a drop rotational motion. This results in a very complicated motion and in the termination of the experiment because of the
inability to control the spot heating location. In order to circumvent this drawback, we have investigated mostly two-dimensional flows in drastically flattened ultrasonically levitated drop and
spot heated at one end. Flattened drops can be created and held in a stable horizontal position for high-viscosity liquids (m > 10 poise). By using suspended tracer particles and laser illumination, we have been able to resolve the three-dimensional flows within the disk-shaped drop. The
flattening of the drop in a plane perpendicular to gravity minimizes the influence of natural buoyancy, and reveals the essential features of thermocapillary flows in the major plane of the drop.
By suppressing in-plane rotational motion with a glass fiber, we have been able to observe the effects of heating raising the local surface temperature in a range between 10
and 100 oC. We have been able to record steady thermocapillary motion in the plane of the flattened drop. Close examination revealed a double set of dipole-like circulation pattern. A higher
velocity (1 to 5cm/s) small vortex is located near the heated spot region, while a larger counter-rotating vortex extends over the majority of the drop cross-section. These measurements were
carried out for fairly low values of the Marangoni between 100 and 250.
Analytical Results
The effects of air compressibility have been accounted for in a greater detail
than in the previous analyses. These include possible influence of the thermal boundary layer and, if
the surrounding gas is a multi-component mixture, of the diffusional boundary layer. Acoustic
streaming around a drop in a liquid, with comparable viscosities inside and outside, has been
considered.
It is well known that acoustic streaming originates in the viscous boundary
layer at the surface of a particle, where flow is genuinely rotational and thus, a steady flow component
is generated as soon as nonlinearities take effect. While this crucial role of the viscous
boundary layer has been widely recognized, little attention has been paid to the role the thermal
boundary layer. The special case of a solid sphere acoustically levitated in a gas medium has been
studied. As solids are generally much more thermally-conductive than gases, the temperature is
practically constant within the solid body while oscillating in a sound wave (consisting of velocity,
pressure and density oscillations). Therefore, a thermal boundary layer is formed at the
surface, in addition to the viscous one. Since density is a function of temperature, a sharp temperature
variation in the boundary layer gives rise to the corresponding density variation. The latter in
turn influences the flow field (continuity equation). In the final analysis, the acoustic streaming
intensity proves to be globally affected.
Furthermore, if we deal with a multicomponent gas, the effect of thermodiffusion
emerges as another factor that may also affect the streaming. The temperature gradient
brings about a concentration gradient, and therefore, as density depends on composition, there
appears an additional contribution to density, and consequently, to the acoustic streaming.
A remarkable new result is that acoustic streaming proves to be much more
intense for the drop-in-liquid system, than for a solid sphere in a liquid. This flow originates
in the external and internal boundary layers at the drop surface. Now that there is the velocity
continuity condition to be satisfied, the acoustic streaming becomes more pronounced with a mobile
interface.
The streaming around a rigid sphere has been studied in the long wavelength
approximation for a general acoustic field of a given frequency. In particular, the acoustic torque
on the sphere has been calculated. Generalizing Riley's (1966) work, we studied the steady
streaming around a motionless sphere with the fluid oscillating with a complex velocity amplitude.
The result for the steady streaming velocity at the outer edge of the boundary layer is found in a
very general form in terms of the velocity amplitude and the oscillation frequency. Since the
oscillation velocity has a general form admitting phase differences with respect to the vector
components, there is, in general, a net torque with an expression given in terms of the velocity
amplitude, the frequency, and the kinematic viscosity.
Sadhal, S.S., Rednikov, A., Ohsaka, K., Ground Based Studies of Thermocapillary Flows in Levitated Laser-Heated Drops, Fifth Microgravity Fluids Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1307-1321, August 9, 2000.