Shiho Tanaka

Oscillatory and chaotic thermocapillary convection in a half-zone liquid bridge

Thermocapillary-driven convection in a half-zone liquid bridge was investigated experimentally. The induced flows were categorized into several regimes mainly through the pattern of the suspended particle motion in the bridge and the surface temperature variation. Special attention was paid to the flow structures far beyond the critical condition. Chaotic and turbulent flows were realized in this configuration. They were distinguished from the periodic oscillatory flow by applying the pseudo-phase-space reconstruction from the time series of the surface temperature variation.

Formation of dynamic particle accumulation structures in oscillatory thermocapillary flow in liquid bridges

We report on the behavior of small particles of dilute concentration in time-dependent (oscillatory) thermocapillary flow in cylindrical liquid bridges. The particles accumulate in a dynamic string for certain aspect ratios of the liquid bridge and at, typically, two times the critical Marangoni number for the onset of time dependence. This was observed for particles with a density larger and smaller than that of the fluid and for the isodense case. If looked at in a snapshot, this string would be wound m times around the thermocapillary vortex as a deformed spiral. If one looked at the full dynamics, it would be seen that the spiral string is rotating around its ring-shaped axis. The phenomenon is called a dynamical particle accumulation structure (dynamical PAS). The mode m is the mode number of the oscillatory flow field with m wavetrains of the hydrothermal wave (HTW) traveling in the azimuthal direction. We visualize and describe the different modes m in detail. We give direct experimental evidence f...

Flow structure and dynamic particle accumulation in thermocapillary convection in a liquid bridge

Thermocapillary convection is induced in a liquid bridge by a nonuniform surface tension distribution caused by an axial temperature difference. A toroidal vortex is formed by the thermocapillary force over the free surface. The induced flow is visualized by using fine particles as tracers. At a sufficiently high Marangoni number, three-dimensional standing and traveling oscillatory flows appear, and under certain flow conditions, the tracer particles form particle accumulation structures (PAS). In the present study, the conditions for the occurrence of PAS have been carefully investigated with focus on the spiral loop PAS (SL-PAS) that appears when the flow exhibits a traveling mode. The particles gather along a closed spiral loop that winds itself around the toroidal vortex. Observed from above, the spiral loop looks as if it is rotating azimuthally. The number of spirals corresponds with the azimuthal wave number of the traveling wave and each spiral consists of either single or double turns. The azimu...

Particle Accumulation Structures in Time- dependent Thermocapillary Flow in a Liquid Bridge Under Microgravity

The formation of a dynamic spiral string of particles with larger density than the fluid was investigated for time-dependent thermocapillary flow in liquid bridges under various gravity conditions including microgravity. The dynamic spiral string forms after approximately 20–60 oscillation periods from the homogeneous dilute particle suspension. It was found that the action of gravity is not decisive in the process of the particle accumulation structure (PAS) but gravity influences the flow field for PAS-formation. We could realize and observe PAS with modal structure m=3 under μ-g but modal structure m=2 occurred only during a transient of the operating parameters in an aspect ratio-range different from that under normal gravity. The correlation of the optically observed PAS structure with the temperature structure of the azimuthally rotating hydrothermal wave on the free surface is the same under microgravity as under normal gravity indicating that PAS is a pure Marangoni effect.

Various flow patterns in thermocapillary convection in half-zone liquid bridge of high prandtl number fluid

Abstract Various flow patterns induced by a thermocapillary-driven convection in a half-zone liquid bridge of a high Prandtl number fluid (Pr = 0(10)) far beyond the critical condition were investigated experimentally. After the onset of oscillatory convection, one can observe several types of flow patterns with increasing a temperature difference between the both end surfaces of the bridge. The flow patterns were categorized through flow visualization, measurement of surface temperature variation and reconstruction of the pseudo-phase space.