Capillary-driven indentation of a microparticle into a soft, oil-coated substrate.

Small scale contact between a soft, liquid-coated layer and a stiff surface is common in many situations, from synovial fluid on articular cartilage to adhesives in humid environments. Moreover, many model studies on soft adhesive contacts are conducted with soft silicone elastomers, which possess uncrosslinked liquid molecules (i.e. silicone oil) when the modulus is low. We investigate how the thickness of a silicone oil layer on a soft substrate relates to the indentation depth of glass microspheres in contact with crosslinked PDMS, which have a modulus of <10 kPa. The particles indent into the underlying substrate more as a function of decreasing oil layer thickness. This is due to the presence of the liquid layer at the surface that causes capillary forces to pull down on the particle. A simple model that balances the capillary force of the oil layer and the minimal particle-substrate adhesion with the elastic and surface tension forces from the substrate is proposed to predict the particle indentation depth.

PMMA-titania floating macrospheres for the photocatalytic remediation of agro-pharmaceutical wastewater.

Antibiotics such as tetracycline are used on a large scale in agriculture, and can become concentrated in wastewater lagoons that are used in conjunction with confined animal feeding operations. Solar-illuminated titanium dioxide can be used to photocatalytically degrade aqueous tetracycline, but its application in a lagoon environment requires that the photocatalyst be supported on a macroscopic support material to prevent loss of the nanoscale photocatalyst into the environment. In this work, titanium dioxide was deposited within a porous poly(methyl methacrylate) film on the surface of floating 7.0 cm diameter acrylic spheres. Six of these floating spheres removed over 96% of the tetracycline in 3.5 L of 60 mg/L tetracycline in natural pond water during 24 hours of solar illumination. The durability of these spheres under long-term solar exposure was also investigated along with the amount of photocatalyst lost from the sphere surface during use. These macroscale floating composite spheres provide a new method for removing tetracycline from wastewater lagoons with minimal risk of being displaced in the environment due to the large size of the spheres.

Creasing in microscale, soft static friction.

Utilizing colloidal probe, lateral force microscopy and simultaneous confocal microscopy, combined with finite element analysis, we investigate how a microparticle starts moving laterally on a soft, adhesive surface. We find that the surface can form a self-contacting crease at the leading front, which results from a buildup of compressive stress. Experimentally, creases are observed on substrates that exhibit either high or low adhesion when measured in the normal direction, motivating the use of simulations to consider the role of adhesion energy and interfacial strength. Our simulations illustrate that the interfacial strength plays a dominating role in the nucleation of a crease. After the crease forms, it progresses through the contact zone in a Schallamach wave-like fashion. Interestingly, our results suggest that this Schallamach wave-like motion is facilitated by free slip at the adhesive, self-contacting interface within the crease.