solid-stabilized emulsions using paramagnetic particles at an oil/water
interface that can undergo macroscopic phase separation upon
application of an external magnetic field. A critical field strength is
found for which emulsion droplets
begin to translate into the continuous-phase fluid. At higher fields,
the emulsions destabilize, leading to a fully phase-separated system.
This effect is reversible and long-term stability can be recovered by
the components with mechanical agitation.
Magnetic_Pickering_emulsion.avi (11.5 MB)
Shape and buckling instabilitites of particle-covered drops
|Dessication or drying of
emulsions drops can occur with either drops attached to surfaces in the
form of pendant or sessile drops, or suspending in air, as in
the case of spray drying. A droplet of liquid encapsulated by a “clean”
interface that is only characterized by a surface tension will be
in size in a manner that is completely described by the Young-Laplace
In the absence of gravity, a suspended droplet will remain spherical
the internal pressure will increase in inverse proportion to the
decreasing drop radius. If the droplet is sessile, it will shrink as a
spherical cap and its contact angle will remain constant. The situation
is completely changed
if the droplet contains a polymer solute or suspended particles, which
be driven to the interface. This can induce a transition from an
that is fluid and only characterized by a surface tension towards a
film that possesses elastic moduli and that can sustain anisotropic
The transition from a fluid film to a solid film is manifested in shape
and buckling of droplets as their volumes are decreased.
We study shape and mechanical instabilities that occur when deflating drops covered by elastic shells.
We are developing a novel method to move and control aqueous drops on superhydrophobic surfaces using magnetic fields. Small water drops (volume 5-15 microliters) that contain fractions of paramagnetic particles as low as 0.1 % in weight can be moved at relatively high speed (7 cm/s) by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This new approach to microfluidics has the advantages of faster and more flexible control over drop movement.
tennis_drop.avi (4.5 MB) splitting_drop_side.wmv (6.8 MB)