Behaviour of Acoustically Levitated Drops in Mid-Water

Abstract

A low-impact acoustic levitation system has been developed to study immobilised single drops in liquid–liquid systems. The ability to observe liquid drops several millimetres in diameter for days enables fundamental research into a wide range of mechanisms. Non-invasive optical measurements with excellent optical accessibility are possible. This experimental work provides the basis for mass transfer studies, emphasizing the precise volume determination, signal noise, reproducibility, and the impact of the acoustic field on the drop and its surrounding environment. The setup can be effectively controlled and proves beneficial for research objectives provided that all liquid phases are entirely degassed, and there are no compressible voids present within the liquids. In addition to the precise, uniform, and reliable measurement conditions, we observed no acoustic streaming in the proximity of the drop and there was no significant vibration of the drop. Qualitative observations using rainbow schlieren deflectometry indicate that the nodal or anti-nodal planes of the standing waves can act as barriers to the dispersion of inhomogeneous dissolved substances in the continuous phase.

Drop Dissolution Intensified by Acoustic Levitation

“…The authors recently demonstrated that the standing wave affects the spread of a substance dissolved inhomogeneously in the continuous phase. Two contrary effects were identified [ 12 ]: For one group of substances, the spread was not affected by the nodal plane above the drop. However, regions of maximum acoustic particle velocity at the outer tip of the horn of the Langevin transducer could not be overcome; For the other group of substances, the nodal planes act as barriers.…”

“…The setup for low-impact acoustic levitation was used to measure immobilised dissolving single drops. The acoustic levitation measurements (ALMs) were conducted following the procedure obtained in [ 12 ]. Alternations to the settings would modify the acoustic environment, potentially leading to different results.…”

“…The concentration of a substance in a liquid determines the refractive index, which can be visualised indirectly with this method. The setup used is described in more detail in [ 12 ].…”

“…To overcome some of the previously stated challenges, a low-impact acoustic levitation system in a liquid–liquid environment is used for further investigations. The developed system was reported recently in [ 12 ]. The ability to immobilise a drop for several days allows non-invasive optical measurements and can be used for a variety of fundamental research applications.…”

“…The objective of this work was to further investigate the characteristics of acoustic levitation in liquid–liquid systems. The influence of acoustic levitation on mass transfer has already been discussed in [ 12 ] and will now be quantified for dissolving drops in binary chemical systems.…”