CAREER: Elastohydrodynamic lubrication of soft patterned interfaces

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Haptic technology creates the perception and experience of touch by applying forces to a user. It is used commonly in applications such as remote surgery and robotics. In some applications, the forces are generated as two surfaces slide over each other, lubricated by a thin film of liquid between the surfaces. This CAREER award focuses on elastohydrodynamic lubrication (EHL), which refers to the special case where the two surfaces are deformable (“elasto”) and the forces on them depend both on their deformation and the flow (“hydrodynamic”) of lubricating fluid between them. In many cases of interest, the surfaces are not smooth, but, instead, have patterns embedded in them. Although the goal of haptic engineering is to produce forces exerted on the length scale of human fingers, the origin of the EHL forces associated with fluid-solid friction on very thin lubricating layers is not well understood, which makes it a challenge to recapitulate the full haptic perception for humans. This project comprises experiments and computer simulations that will determine the various forces that occur in EHL for well-defined model geometries that contain the essential features of haptic applications. The results of the work will aid practitioners in the design of haptic technology that more realistically reproduces sensations of touch. In addition, the project will include activities to stimulate interest in fluid mechanics and soft matter among the public through interactive citizen science sessions, through virtual perception demonstrations for science camp participants, and through instruction in haptic applications and soft matter for undergraduates.A set of experiments will be conducted to test the central hypothesis that the shear, bending, and compression of two surfaces are decoupled by patterns with different spatial dimensions. The proposed research will address three outstanding questions in EHL tribology: 1) Is it possible to sense complex rheology using surface patterns? 2) Is it possible to measure film thickness in soft tribopairs during sliding? 3) How does the bending of patterns influence film thickness? Experiments will be designed to characterize the friction of patterned soft materials such as elastomers and hydrogels. Tribo-rheometry and a haptic device will be used to correlate tribology to forces and torques measured with human hands. Lubrication hydrodynamics and direct visualization will be used to model the film thickness between tribopairs. The effects of sliding orientation, bending, and surface topography on fluid flow profiles will be investigated using a combination of experiments, scaling theory, and computational fluid dynamics simulations. Constitutive models for non-Newtonian fluids will be used to compute the stress tensor and fitted to the experimental data. These research aims will provide the analytical foundation for haptic engineering involving liquids.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.