Magnetic field based actuation and amalgamation of ferrofluid droplets on hydrophobic surface: An experimental and numerical study

Abstract

This article presents a detailed experimental analysis along with numerical simulations to provide the information about shape evolutions and mechanism of actuation and amalgamation of the ferrofluid droplets deposited on a hydrophobic surface by moving a permanent magnet. To validate the numerical methods used in this article, a benchmark phenomenon of a sessile droplet spreading under the effect of a non-uniform magnetic field is first simulated, and the results are compared with available experimental observations. To further ensure the accuracy of experimental and numerical techniques and to understand the wetting properties and spreading behavior of non-stationary ferrofluid droplets, a prototype demonstration of the merging droplets on a hydrophobic solid surface in the presence of a permanent magnet is designed. It is observed that for hydrophobic surfaces, the merging droplets entrap an air bubble at the time of first contact. Moreover, the physics behind the transient variations of droplet morphology and the effect of the state-of-the-art parameters on droplet actuation are also discussed. The force evaluation, energy variations, velocity contours, and velocity vectors of the moving droplet are provided to understand the internal behavior of droplet mobility. Experiments are performed several times with different speeds of moving a magnet to find the critical velocity when the droplet fails to follow the magnet motion. While doing so, we encounter an anomalous phenomenon of thread formation and daughter droplet generation at the receding end of the sliding droplet. A phase diagram is also provided in the end, which describes different regions of the sliding phenomenon.