(728g) The Effect of the Surface Roughness On Floating Hydrophobic, Micron-Sized Particles At An Oil/Water Interface

A spherical particle placed at an oil/water interface (oil above water),which is more dense than either fluid phases, deflects the interface downward as gravity acts to push the particle through the surface. In spite of its weight, the particle can remain suspended on the interface if the interfacial tension or capillary force created by the interfacial deflection is large enough to support the buoyant weight of the particle. The maximum particle size which can be balanced by the capillary force is a function of the interfacial tension and the contact angle at the particle surface (defined as measured through the water phase). The larger the contact angle (or the greater the surface hydrophobicity), the greater is the maximum particle size,which can be supported. Atomically smooth surfaces are characterized by contact angles which are determined by the surface chemistry. However, recent research has shown that the contact angle of water on hydrophobic textured surfaces in air (as measured through the water phase) can be increased significantly relative to the corresponding value on a smooth surface with the same hydrophobic chemistry as that of the texture. This superhydrophobic effect can change the smooth surface contact angle on a hydrophobic surface from 100 degrees to 160 degrees or more, and is due to the pinning of the interface onto the texture as the water does not saturate into the texture (Cassie-Baxter wetting). In the case of a water droplet placed on a hydrophobic,textured surface immersed in oil, this same effect can result in an increase in the contact angle of the water droplet in the oil. This raises the possibility that the size of hydrophobic textured particles,which can be supported at an oil/water interface, is significantly larger than smooth particles with the same surface chemistry because of the increase in the contact angle.

In this investigation, we demonstrate this effect by using silica particles that have been textured by the binding of hydrophilic nanoparticles. In the case of both the smooth and nanoparticle textured surfaces, the surfaces are made hydrophobic by silanization. Experi- ments on planar substrates of smooth silica surfaces and those with the nanoparticles, after silanization, demonstrate the increase in contact angle of water drops placed on the sur- face. We also verify that larger surface textured spherical particles can be supported at the oil/water interface relative to smooth particles with the same (silanated) surface chemistry. We also examine the effect of the interfacial tension of the oil/water interface on the relative difference between the maximum particle size supported by smooth and microtextured par- ticles. The tension is varied by introducing a surfactant into the continuous oil phase. We show that even with the reduction in the tension due to the surfactant adsorption onto the interface, textured particles of considerable size can be supported at the oil/water interface due to the increase in contact angle at the textured surface.