Capillary Interactions in Wetting: Rotation of Particles at Interfaces and Removal of Particles by Drops
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Abstract
In this thesis, I have experimentally and theoretically investigated the mechanism
by which a liquid drop removes a single solid particle from a surface.
To address this problem, I designed a method based on laser scanning confocal
microscopy to image drop-particle collisions dynamically (speeds 10 to 10 000 µm/s)
and to measure the horizontal force acting on the drop during these collisions. Water
drops, glass particles and crosslinked polydimethylsiloxane surfaces were used in most
of the experiments.
Two main collision outcomes were observed: either the particle collided with
the drop and remained attached to the liquid-air interface (successful removal), or
the particle entered and exited the drop (unsuccessful removal). The viscous force
measured when the particle moved through the drop was negligible compared to the
capillary force acting on the particle when it was attached to the liquid-air interface.
Consequently, the dominant force that determines particle removal is the capillary
force. A liquid-air interface will successfully remove a particle when the capillary
force exceeds the resistive force that the particle has to overcome to move (roll/slide)
on the surface.
To understand the difference between the forces acting on a rolling particle and
a sliding particle, I theoretically modelled the capillary force for both cases. There
are two main results. Firstly, a rolling particle enters a drop more easily than a non-rolling particle (up to 40% less force is required). Secondly, a particle experiences
a resistive capillary torque when rolling at an interface. This torque significantly
increases the rolling resistance of the particle.
The theoretical model for the resistive capillary torque is directly applicable in
addressing broader questions on the motion of particles at interfaces and the mobility
of moist granular matter. Moreover, the experimental method presented in this thesis
can be applied to study a variety of problems in the field of wetting (and beyond),
where the combination of microscopic imaging and friction force measurements is
often insightful.