We prepare 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 remixing 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 reduced in size in a manner that is completely described by the Young-Laplace equation. In the absence of gravity, a suspended droplet will remain spherical and 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 can be driven to the interface. This can induce a transition from an interface that is fluid and only characterized by a surface tension towards a solid film that possesses elastic moduli and that can sustain anisotropic stresses. The transition from a fluid film to a solid film is manifested in shape transitions 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. slow_drop.avi  (5.2 MB)                                               drop_in_circles1.wmv (1.6 MB)                           tennis_drop.avi (4.5 MB)                                              splitting_drop_side.wmv (6.8 MB)

• Silicon Nanowire and Polyethylene Superhydrophobic Surfaces for Discrete Magnetic Microfluidics, Appl. Surf. Sci. 254 (2007) 330-334 
• Magnetic movement of biological fluid droplets, J. Magn. Magn. Mater. 311 (2007) 238-243
• Discrete magnetic microfluidics, Appl. Phys. Lett. 89 (2006) 34106(1-3)
• Shape and buckling transitions in solid-stabilized drops, Langmuir  21 (2005) 10016-10020 
• Pickering emulsions with controlable stability, Langmuir  21 (2005) 2158-2162
  
• Particle-laden interfaces: Rheology, Coalescence, Adhesion and Buckling at "Colloidal particles at liquid interfaces", by Bernard Tommy S. Horozov. (ISBN-13 978-0-521-84846-6)