Tunable Friction Through Stimuli Responsive Hybrid Carbon Microspheres.

We explore the use of poly(N-isopropylacrylamide) (PNIPAm)-grafted carbon microspheres (CM) dispersed in water as a stimulus-responsive lubricant. A critical concentration between 3 and 5 mg/mL of PNIPAm-grafted CM is needed to achieve low friction (coefficient of friction ∼ 0.04) at room temperature between borosilicate and silicon surfaces. An increase in the temperature of the system above the lower critical solution temperature (LCST) causes the aggregation of PNIPAm-grafted CM which leads to an increase in friction forces. The process is not immediately reversible unless the lubricant is sonicated so as to redisperse the aggregates. This work provides insight into the rolling friction mechanism and demonstrates the importance of particle singlets in achieving effective lubrication through a rolling mechanism.

Anomalous Potential-Dependent Friction on Au(111) Measured by AFM.

We present an exploratory study of the tribological properties between an AFM probe and a Au(111) surface in an aqueous environment while subjected to applied surface potentials. Using a three-electrode setup, the electrical potential and interfacial electric field on a Au(111) working electrode are controlled. Lateral force microscopy is used to measure the friction forces between the AFM probe and the Au surface. As the AFM probe approaches the surface, normal forces are also measured to gain insight into the interfacial forces. When a positive potential is applied to the Au surface, the friction is found to rise sharply at a critical potential and level off at a relatively high value. However, when a negative potential is applied, the friction forces are low, even lower compared to the open circuit potential case. These changes in friction, by a factor of approximately 35, as a function of the applied potential are found to be reversible over multiple cycles. We attribute the origin of the high friction at positive potentials to the formation of a highly confined, ordered icelike water layer at the Au/electrolyte interface that results in effective hydrogen bonding with the AFM probe. At negative potentials, the icelike water layer is disrupted, resulting in the water molecules acting as boundary lubricants and providing low friction. Such friction experiments can provide valuable insight into the structure and properties of water at charged surfaces under various conditions and can potentially impact a variety of technologies relying on molecular-level friction such as MEMs.

Enhanced Adhesion of Mosquitoes to Rough Surfaces.

Insects and small animals capable of adhering reversibly to a variety of surfaces employ the unique design of the distal part of their legs. In the case of mosquitoes, their feet are composed of thousands of micro- and nanoscale protruding structures, which impart superhydrophobic properties. Previous research has shown that the superhydrophobic nature of the feet allows mosquitoes to land on water, which is necessary for their reproduction cycle. Here, we show that van der Waals interactions are the main adhesion mechanism employed by mosquitoes to adhere to various surfaces. We further demonstrate that the judicious creation of surface roughness on an opposing surface can increase the adhesion strength because of the increased number of surface elements interacting with the setae through multiple contact points. Although van der Waals forces are shown to be the predominant mechanism by which mosquitoes adhere to surfaces, capillary forces can also contribute to the total adhesion force when the opposing surface is hydrophilic and under humid conditions. These fundamental properties can potentially be applied in the development of superior Long Lasting Insecticidal Nets (LLINs), which represent one of the most effective methods to mitigate mosquito-transmitted infectious diseases such as Malaria, Filaria, Zika, and Dengue.

Polymer grafted hard carbon microspheres at an oil/water interface.

Pickering emulsions offer an established method of stabilizing oil-in-water emulsions as either an alternative to surfactants or as an additive together with surfactants, providing greater colloidal stability even at low particle concentrations. This work presents a novel experimental approach to study the influence of several system parameters on the effectiveness of Pickering emulsion systems. Specifically, a dodecane oil drop stabilized by hard carbon microspheres in an aqueous saline solution is used as a model system to obtain both quantitative and qualitative information on the effectiveness of the microspheres as a function of their surface wetting properties. The test setup, in which a macroscopic oil drop is brought into contact with a test surface in a controlled motion and environment, allows for several aspects of the test (for e.g., oil drop size, approach velocity, normal force, solution ionic strength, temperature, pH, and presence of surfactants) to be potentially controlled and studied precisely. To demonstrate the capabilities of the experimental set-up, hard carbon microspheres are modified with a poly(styrenesulfonate) shell through ATRP in order to tune the wettability of the particles through choice of polymer, which are then used to stabilize a dodecane oil drop in an aqueous saline solution. The particles effectively form a steric barrier preventing the spreading of an oil drop on hydrophobic surfaces and also preventing the coalescence of stabilized oil drops.