In modeling complex mechanisms such as mass transfer and mass transfer phenomena, the choice of assumptions and boundary conditions is difficult. For this reason, first, simplified cases are investigated and validated. However, the developed models often have significant limitations since reliable experimental validation data for the simplified cases are rare.
Currently, studies of motionless droplets in liquid/liquid systems are performed by exposing the droplets to a flow field with droplets locally fixed either with capillaries, by using chemical systems with no density difference, or supressing gravity. With the latter methods experiments are significantly limited by the choice of chemical system and the costs incurred. The first two methods are widely used, but experimental conditions have a significant influence on the energy, momentum, and mass transfer between disperse and continuous phases.
In this project, a novel setup is developed to study mass transfer and mass transfer phenomena on free-floating single droplets in liquid/liquid systems without external influences. For this purpose, acoustic levitation is used to fix single droplets without contact, which can then be analyzed non-invasively with optical measurement techniques (see figure above). For acoustic levitation of a droplet, a field of standing ultrasonic waves is generated in a measurement environment to counteract the forces caused by buoyancy or gravity. Although acoustic levitation of single droplets in liquid/liquid systems has been used for various studies in the 1980s and 1990s, surprisingly, there is currently no application in research of mass transfer and mass transfer phenomena.
In this work, an acoustic measurement cell was developed using water as the continuous phase. Many solvents or liquids that are not miscible with water can be used as a disperse phase. Droplets with diameters from 1 to 6 mm can be levitated in the cell, with contact times from a few seconds to several days. Serial studies of free-floating single droplets are possible with an automated injection and aspiration system. One observation axis is occupied by a high-speed camera, the second axis can be occupied by any other optical measurement method. Based on the high-speed images, the transient droplet volume is determined for detailed integral mass transfer observations at the droplet interface.
Initial investigations were carried out in two standard chemical systems recommended by the "European Federation of Chemical Engineering". The systems are n-butyl acetate/acetone/water and toluene/acetone/water. The second system tends to the so-called Marangoni effect, which ultimately occurs due to the spontaneous compensation of gradients in the interfacial tension at the interface, caused by mass transfer. These abrupt processes lead to movements of the individual droplets (see example video on the right) and have a considerable influence on mass transport.
From the transient volume data, the volume fraction of the transfer component acetone can be calculated in the ternary system (figure below). Due to the highly detailed of the volume measurement carried out, the smallest amplifications (∆Φ in figure below) in mass transfer caused by individual Marangoni eruptions can be determined quantitatively.
Pending studies aim to quantify the influence of the acoustic field on mass transfer in the binary and ternary systems. Based on the results, models are to be developed so that this influence can be extracted from the measurements. Consequently, measurements on free-floating single droplets without external influences in liquid/liquid systems will be possible.