Frequency-Dependent Magnetic modulation of deposition morphology

Abstract

This paper presents a novel approach for magnetic modulation of deposition morphology in an evaporating ferrofluid droplet. The magnetic field strength and ferrofluid concentration are kept unchanged, while the actuation frequencies are varied from 0.016 Hz to 5 Hz. In the absence of a magnetic field, a coffee-ring formation is observed and consistent with previous studies\cite{deegan1997capillary,deegan2000contact,saroj2019drying}. The application of a time-dependent magnetic field significantly modifies the deposition morphology. The periodic magnetic field induces the formation of multiple concentric rings during evaporation. The number of rings initially increases with increasing actuation frequency of the electromagnet. However, beyond a critical actuation frequency ($f_c = 0.2\,\text{Hz}$), the number of rings decreases. At higher actuation frequencies, magnetic particles preferentially deposit in the central region of the droplet, resulting in suppression of the coffee-ring effect. Additionally, the thickness of the inner rings and the ring spacing decrease with increasing actuation frequency up to critical actuation frequency. The transition from multi-ring formation to coffee-ring suppression is governed by the competition among magnetic forcing, capillary flow, and particle diffusion. The underlying physical mechanisms responsible for droplet dynamics and deposition morphology under periodic magnetic fields are evaluated using scaling arguments. The results demonstrate that diffusive particle transport plays a dominant role in determining the deposition pattern. A non-dimensional magnetic switching number, based on the magnetic perturbation timescale, is introduced as a control parameter to characterize the frequency-dependent deposition behavior.

Figures (8)

• Figure 1: Sketch of (a) experimental arrangement and (b) shows the ON and OFF of electromagnet with time.
• Figure 2: Variation in height during evaporation of the droplet: (a) without application of the magnetic field and (b) with actuation of the magnetic field at $f$ = 0.016 Hz and $f$ = 0.05 Hz. The side view images (top of the figure) show the increased height during switch on state and switch off states of the magnet during magnetic actuation. The inset color images in figure (b) show the magnetic flux density (B) distribution inside the droplet by COMSOL Multiphysics.
• Figure 3: Variation of the diameter of the droplet with evaporation time for without magnetic field actuation and with magnetic field actuation. .Here, $\rm{d_{ON}}$ and $\rm{d_{OFF}}$ are diameters of the droplet during ON and OFF of electromagnet, respectively.
• Figure 4: Deposition pattern along with the variation in gray value of drying droplet at different switching frequencies.
• Figure 5: Illustration of the chain formation and its breakup corresponding to magnet ON and magnet OFF, respectively.