Effect of external magnetic fields on the contact angle of magneto-responsive fluids

The increasing demand of technological devices and fabrication thereof has led to the development of so-called smart materials (SMs). SMs can change their chemical and physical properties upon various external stimuli (electric, mechanical, magnetic, thermal, luminous). The use of magnetoresponsive fluids could enable novel technological applications if it were possible to control the contact angle by tuning the amplitude and gradient of an external magnetic field. In this contribution, we focus on the magneto-response of two separate systems: a ferrofluid based on γ-Fe2O3 and magnetic ionic liquids (MILs, i.e. liquid salts at room temperature where one of the ions is magnetic, such as iron, cobalt, nickel). γ-Fe2O3 ferrofluids have been extensively investigated in the literature and some studies reported that their wettability (i.e. contact angle) varies under magnetic field gradients. For what concerns MILs, several studies reported their synthesis and their characterization, but their wettability under a magnetic field has not been investigated yet. However, contact angles depend on the balance between surface tension, i.e. those of the substrate, of the fluid drop being investigated, and on the other surrounding phase (be it vacuum, air or a second liquid). Hence, in this study we also investigate the effect of a second immiscible liquid on the wettability of γ-Fe2O3 ferrofluid and MILs. Lastly, we correlate the contact angle measured experimentally with the magnitude of the magnetic field gradient generated in our custom-made setup, as reconstructed by Hall probe 3D-mapping.