Unveiling combined thermal and magnetic field effects in ferrofluid marble evaporation: An experimental exploration

Abstract

Ferrofluid marbles, formed by encapsulating a ferrofluid core with a hydrophobic nanoparticle shell, offer high mobility and environmental isolation while enabling noncontact, remote manipulation through an external magnetic field. These characteristics make them a suitable model system for investigating the coupled effects of evaporation, magnetic forcing, and morphological deformation. This study systematically investigates the evaporation dynamics of ferrofluid marbles under steady vertical magnetic fields and varying substrate temperatures, with comparisons to de-ionized-water marbles. The hydrophobic nanoparticle shell significantly retards evaporation compared with bare ferrofluid droplets, while in the absence of a magnetic field, ferrofluid marbles evaporate slightly faster than water marbles, consistent with their higher thermal conductivity. Application of a steady magnetic field slows the evaporation rate, with the effect being more pronounced at lower substrate temperatures and increasing the total lifetime by up to 20.8%. The magnetic field induces vertical elongation of the marble, reducing the effective surface area for evaporation and altering the internal fluid dynamics. This manipulation also controls the final deposition pattern, transitioning from flat (without field) to peaked (with field). The findings provide new insights into the control of evaporation dynamics in ferrofluidic marbles, with implications for digital microfluidics, drug delivery, and surface patterning.