Premature Rayleigh jets in a millimeter-scale water drop levitated in a strong electric field

Abstract

Soft matter electrostatic levitation (SEL) is a containerless processing technique that plays an important role in investigating different challenging areas of physics. An area of interest is the study of crystallization and nucleation theory. The measurement of the thermophysical properties of solutions at high supersaturation levels is extremely challenging because of the heterogeneous nucleation sites, such as the container walls that initiate crystallization at low supersaturation levels. The SEL was developed at Iowa State University (ISU) in collaboration with the Korea Research Institute of Standards and Science (KRISS). This technology levitates colloidal suspensions, sodium sulfate, and other solutions. The levitation is achieved when charged liquid drops are freely suspended in the air by the electrostatic field generated between a pair of vertical electrodes. This non-contact environment allows the samples to levitate without contamination by container walls. The samples are allowed to evaporate and crystallize at high supersaturation levels. The measurements of thermophysical properties at high supersaturation levels can be achieved. However, a different challenge was observed during the experiments, where the charged samples were found to jet during the levitation. This phenomenon causes the loss of the samples. Furthermore, the information on the thermophysical properties during the jetting process still needs to be discovered. Jetting is a phenomenon where the surface of the charged drops breaks up into an elongated cone shape, or sometimes a filament or drops fly out of the surface. The experiments conducted on colloidal suspensions with the SEL found that a filament with a cylindrical shape was being thrown off the top surface of the samples. However, a Taylor cone was observed for experiments on deionized water. This instability is an obstacle to achieving the thermophysical properties’ measurements at high supersaturation levels. As aresult, this study focuses on investigating the jetting phenomenon. Rayleigh described this phenomenon in 1882, where the model describes the isolated charged drops. In addition, in 1924, Taylor derived a jetting model for uncharged drops in an electric field. This current work uses deionized water (DI water) to study the jetting phenomenon. All the samples were jetting below the Rayleigh and Taylor limits, and this phenomenon was called a premature jet. Also, all the samples were found to have a continuous charge loss during the levitation. The continuous charge loss and jetting below the limits contradict the Rayleigh theory. As a result, this work will likely identify the mechanism of the premature jet and the continuous charge loss. Finally, an oscillation method will be applied to the levitated drops to measure the surface tension of the samples to show how the surface charge and the external electric field affect the effective surface tension.