Magnetic suppression of the coffee ring effect

Abstract

This paper reports the evaporation and drying characteristics of ferrofluid droplets on a PDMS (Polydimethylsiloxane) substrate under the influence of magnetic field. The effect of magnetic field strength and asymmetry of the external magnetic field with respect to the axis of the droplet is studied. Goniometric measurement is used for visualization of the evaporating droplets. Confocal microscopy is used for imaging the drying pattern and particle motion inside the droplet. The magnetic field strength is controlled by varying the distance between the magnet and the substrate. COMSOL Multiphysics software is used to report the magnetic field and magnetic force distribution. The uniform deposition of drying pattern is observed under the influence of magnetic field. The effect of magnetic field on evaporation dynamics is attributed to the modification in drying pattern. The evaporation of the droplet takes place in constant contact radius (CCR) mode, constant contact angle (CCA) mode, mixed mode, and late pinning mode. Both sliding of the droplet and receding of the contact line is observed during evaporation when the center of magnet is not aligned with the center of the droplet. The suppression in duration of the CCR and CCA modes of evaporation and increase in duration of mixed mode evaporation is observed at higher magnetic field strength. The initial contact angle decreases and receding contact angle increases with increasing strength of magnetic field. This is attributed to lower concentration of nanoparticle in the contact line region resulting in lower frictional resistance to the contact line motion. Hump formation takes place at the apex region of the droplet, which depends on the strength of the magnetic field and local concentration of nanoparticle. The hump formation is followed by splitting of the droplet.