Abstract
Fluid drops coated with particles, so-called Pickering drops, play an important role in emulsion and capsule applications. In this context, knowledge of mechanical properties and stability of Pickering drops are essential. Here we prepare Pickering drops via electric field-driven self-assembly. We use direct current (DC) electric fields to induce mechanical stress on these drops, as a possible alternative to the use of, for example, fluid flow fields. Drop deformation is monitored as a function of the applied electric field strength. The deformation of pure silicone oil drops is enhanced when covered by insulating polyethylene (PE) particles, whereas drops covered by conductive clay particles can also change shape from oblate to prolate. We attribute these results to changes in the electric conductivity of the drop interface after adding particles, and have developed a fluid shell description to estimate the conductivity of Pickering particle layers that are assumed to be non-jammed and fluid-like. Retraction experiments in the absence of electric fields are also performed. Particle-covered drops retract slower than particle-free drops, caused by increased viscous dissipation due to the presence of the Pickering particle layer.
Highlights
Particles bound to drop interfaces by capillary forces can form a protective layer, preventing drop coalescence
The electric field-induced deformation of a pure silicone oil drop is compared with the deformation of a silicone oil drop covered by PE particles and a silicone oil drop covered by clay particles. At this field strength (250 Vmm−1 ), the drop covered with PE particles (Figure 1b) is more deformed than the pure drop (Figure 1a), while the drop covered by clay particles (Figure 1c) is less deformed than the pure silicone oil drop
We have shown that the deformation of pure and particle-laden drops can be controlled by varying the strength of the applied electric field and the drop size, and that the deformation magnitude and sign depends on the electrical conductivity of the drop interface
Summary
Particles bound to drop interfaces by capillary forces can form a protective layer, preventing drop coalescence. This constitutes the basic mechanism for Pickering emulsion stabilisation [1,2,3]. External stresses can be applied by various means to study the mechanical and rheological properties of Pickering drops. Buckling has been measured by drop volume compression and expansion [15,16,17], and drop deformation and elasticity have been investigated by mechanical compression [18,19,20], hydrodynamic shear flow [21,22], and microfluidic focusing devices [23]
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