Non-Contact Measurement of Thermophysical Properties of Liquids by Acoustic and Electrostatic Levitation

The thermodynamic behavior of liquids in undercooled states is an area that has gained considerable importance in various applications. In nature, as well as with industrial situations such as liquid-metal spray deposition, liquid states are found to exist below the freezing point, especially if a pure liquid remains in a relatively undisturbed state. Since a disturbance can easily disrupt the undercooled state, measurements need to be carried out non-intrusively. Furthermore, the liquid sample has to be held without a container since most solid containers would promote heterogeneous nucleation at the freezing point. Therefore, electrostatic and acoustic levitation techniques are being employed. The measurement of viscosity and surface tension are being conducted by observing the response of initially deformed drops (acoustically and electrostatically) to more spherical shapes upon relaxation of the deforming force fields. The measurement of properties, such as thermal diffusivity, are conducted by the application of a thermal stimulus to a levitated liquid sample and the observation of the response from which such properties can be inferred. The effect of buoyancy-driven convection is suppressed by horizontally flattening the drop with an acoustic field, thus leaving a small gravitational force potential. This approach has been found to effectively reduce the flow to a thermoacapillary dominant type, accompanied by acoustic disturbances. This lends itself to measurements of the thermocapillary flows with minimal buoyancy interference even in a unit-gravity field.