Shear-induced diffusion: the role of granular clusters

This paper is concerned with the physical mechanisms controlling shear-induced diffusion in dense granular flows. The starting point is that of the granular random walk occurring in diluted granular flows, which underpins Bagnold’s scaling relating the coefficient of self-diffusion to the grain size and shear rate. By means of DEM simulations of plane shear flows, we measure some deviations from this scaling in dense granular flows with and without contact adhesion. We propose to relate these deviations to the development of correlated motion of grains in these flows, which impacts the magnitude of grain velocity fluctuations and their time persistence.

Micromanipulation System for Isolating a Single Cryptosporidium Oocyst

In this paper, an integrated system for contact micromanipulation of Cryptosporidium oocysts is presented. The system integrates five actuators and a partially automated control system and contacts the oocyst using a drawn glass end effector with tip dimensions of 1 μ m. The system is intended to allow single cell analysis (SCA) of Cryptosporidium—a very harmful parasite found in water supplies—by isolating the parasite oocyst of 5 μ m diameter in a new environment. By allowing this form of analysis, the source of Cryptosporidium can be found and potential harm to humans can be reduced. The system must overcome the challenges of locating the oocysts and end effector in 3D space and contact adhesion forces between them, which are prominent over inertial forces on this scale. An automated alignment method is presented, using the Prewitt operator to give feedback on the level of focus and this system is tested, demonstrating alignment accuracy of <2 μ m. Moreover, to overcome the challenge of adhesion forces, use of dry and liquid environments are investigated and a strategy is developed to capture the oocyst in the dry environment and release in the liquid environment. An experiment is conducted on the reliability of the system for isolating a Cryptosporidium oocyst from its culture, demonstrating a success rate of 98%.

Investigation of the Effect of Long-Term Contact on Adhesion Force in Polycrystalline MEMS Devices

The effect of long-term contact on adhesion force between MEMS silicon surfaces is investigated. A test structure is developed that can initiate the contact between the two surfaces without actuation. Therefore, the surfaces may remain in contact for a long period of time. The contacting surfaces are separated using a thermal actuator and the adhesion force is determined utilizing FEM simulations. Freshly released and long-time stored devices were tested, and their adhesion force is determined. The test results show that the adhesion force for devices in contact for a long time is drastically larger than those with short-term contact. Adhesion force values as large as 2.9 μN for long-term contact and as small as 0.4 μN for fresh contact were measured. The large adhesion force associated with long-term contact is believed to be attributed to the formation of native oxide at contacting nano-asperities and formation of covalent bonds between the surfaces, requiring larger force to break the bonds.

FRICTION AND ADHESION OF MONOMOLECULAR COATINGS

The paper presents data on measurements of friction and adhesion forces of silicon ball and silicon plate covered by DDPO4 (dodecylphosphoric acid ester), ODPO4 (octadecylphosphoric acid ester), SEBS (poly[styrene-b-(ethylene-co-butylene)-b-styrene]) and OTS (octadecyltrichlorsilane) and others. Tests were performed with reciprocating microtribometer and contact adhesion meter. The experimental results have shown, that coefficient of friction, between a silicon ball and silicon substrates modified by the monomolecular films, are varied in the range 0.05…0.25, with wear-resistance from 20 to 2000 cycles. An ambiguous correlation was found between friction and contact adhesion forces of investigated surfaces.