Drying pattern and evaporation dynamics of sessile ferrofluid droplet on a PDMS substrate

Abstract

The present study reports the evaporation dynamics, deposition pattern, effect of magnetic field and mechanism behind the pinning or depinning of the contact line of sessile ferrofluid droplet on a PDMS substrate. The concentration of ferrofluid has been varied as 0.0%, 0.1%, 1.0%, and 5.0%. Optical profilometry, confocal microscopy, micro-PIV and goniometry measurements have been carried out. The average evaporation rate increases with increase in ferrofluid concentration i.e. about twice increase in evaporation rate is observed at 5% ferrofluid concentration compared to 0.0% concentration. Crack formation takes place inside the coffee ring of the drying pattern. The crack size i.e. length and width increase with increase in ferrofluid concentration, which can be correlated to the drainage of solvent by the capillary action and formation of cluster. The contact line motion is a function of ferrofluid concentration, which influences the diameter of the coffee ring. The width of the coffee ring, the contact line velocity of the droplet during the receding phase and total drying period decrease with increase in concentration of ferrofluid. The contact line stops receding if the frictional force exceeds the horizontal component of surface tension force at a critical particle density in the contact line region. The application of magnetic field leads to uniform deposition pattern of the drying droplet, which can be correlated to higher duration of mixed mode evaporation of the ferrofluid droplet.