Magnetophoretic Control of Diamagnetic Particles Inside an Evaporating Droplet

The
present study reports the magnetophoresis of diamagnetic particles
in an evaporating ferrofluid droplet. Both solid and ring magnet arrangements
are used to investigate the effect of magnetic field distribution.
The distance of the magnet from the droplet is varied to study the
effect of magnetic field strength. The magnetic field distribution
is computed using COMSOL multiphysics software. Magnetometer measurements
have been carried out to validate the simulation results. The motion
of particles and the drying pattern of evaporating ferrofluid droplets
are visualized using the confocal microscopy technique. Both bright-field
and fluorescence imagings have been carried out to observe the differential
deposition of the fluorescent particle (microparticle) and magnetic
nanoparticles in the absence and presence of a magnetic field. The
velocity of diamagnetic particles as a function of magnetic field
distribution and strength has been studied using the micro-PIV technique.
In the absence of the magnetic field, a ring-shaped deposition pattern
is observed. The mixture of microparticles (diamagnetic) and nanoparticles
(magnetic) is deposited between the outer and inner edges of the ring.
The diamagnetic particles occupy the inner and outer edges of the
ring. Magnetic particles travel toward the higher magnetic field zone
and diamagnetic particles move toward the smaller magnetic field zone
when a magnetic field is applied by a solid magnet placed over the
droplet. This can be attributed to the negative magnetic force originating
from the difference between the susceptibility of magnetic and nonmagnetic
particles. The negative magnetic force on the microparticle increases
as the magnetic field intensity increases, causing the microparticle
to convect faster toward the contact line. The deposition behavior
can be reversed or suppressed using a ring magnet in place of a solid
magnet. In this case, the negative magnetic force is stronger at the
contact line region of the droplet and decreases as it approaches
the center region of the droplet. The deposition behavior of diamagnetic
particle depends on the balance between the Marangoni force and the
magnetophoretic force. Overall, the present study demonstrates the
capability of the controlled deposition of diamagnetic polystyrene
particles by suitable arrangement of the solid and ring magnet.