Uncovering friction dynamics using hydrogel particles as soft ball bearings

Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontriv...

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Bibliographic Details
Published in:Soft matter 2020-04, Vol.16 (15), p.3821-3831
Main Authors: Rudge, Raisa E. D, van de Sande, Jesse P. M, Dijksman, Joshua A, Scholten, Elke
Format: Article
Language:English
Subjects:
Ball bearings
Confined spaces
Deformation
Emulsions
Formability
Friction
Granular materials
Hydrogels
Lubricating properties
Particulates
Pastes
Sliding
Surface properties
Velocity
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Summary:Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials. Hydrogel microparticles transition through four rate-dependent regimes.
ISSN:1744-683X
1744-6848
DOI:10.1039/d0sm00080a