Magnetically Induced Ferrofluid Droplet Manipulation on Slippery Surfaces

Ferrofluid (FF) droplets can be manipulated without direct mechanical contact, but reproducible deformation on conventional hydrophobic or superhydrophobic substrates is often limited by contact-line pinning and nanoparticle contamination. Here, we investigate magnetic-field-induced deformation and presplitting of FF droplets on a nonmagnetic slippery liquid-infused porous surface (SLIPS). The effects of magnet geometry, droplet volume, particle mass fraction, and droplet-magnet distance were examined using side-view imaging and image-based profile analysis. As the magnet approached the SLIPS, the apparent contact angle and base width decreased, whereas the droplet height increased, followed by strongly nonlinear deformation near the presplitting stage. Droplet morphology depended on both magnetic field intensity and field gradient. Among the tested parameters, FF particle mass fraction had the strongest influence on deformation mode, while droplet volume mainly caused proportional geometric scaling. Larger magnets generated stronger fields and steeper gradients, extending the effective actuation range and increasing the splitting distance. Finally, symbolic-regression models were developed to predict normalized droplet height and width, achieving R2 values of 0.94 and 0.89, respectively. The resulting empirical correlations describe presplitting FF droplet deformation on SLIPS within the tested parameter range.