Dynamics of ferrofluidic droplets of chosen geometry

This research project studies drop contact lines dynamics as a function of its geometry. Liquids naturally tend to reach a spherical shape. The proper tuning of drop geometry is achieved by use of ferrofluidic droplets with cores made of magnets of controlled shape. Ferrofluids are colloidal liquids made of nanoscale ferromagnetic particles suspended in a carrier fluid, usually an organic solvent. They find various applications in devices such as speakers and induction brakes, and were initially invented by NASA in the 60’s to drive fluids in a weightless environment.

The ferrofluidic droplets of chosen geometry were characterized by sliding down an inclined plane of controlled slope. Terminal drop speed gives access to damping mechanisms, localized mostly at the contact line. Film deposition also characterizes drop dynamics and exists above a speed threshold related to Landau-Levitch transition. We found that film deposition depends on specific drop geometry. Moreover, a typical droplet would go down the line of greatest slope. We found that fluid recirculation may induce divergence from this natural line at a given angle (see figure 1). The divergence angle was found to be related to the drop asymmetry. This may be further used as a passive microfiltering device for geometry quality control.

Figure 1- Droplet of different geometries experience various contact line dynamics. This impacts the final equilibrium trajectory of a sliding droplet of given shape. Symmetrical shapes (round, square) follow the line of greatest slope on an inclined plane, whereas asymmetrical shapes (“tetris”-like S and Z) diverge from this natural line.

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