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Research Papers: Mixed and Boundary Lubrication

Superlubricity in Gemini Hydrogels

[+] Author and Article Information
Angela A. Pitenis, Juan Manuel Urueña, Andrew C. Cooper

Department of Mechanical and
Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

Thomas E. Angelini

Department of Mechanical and
Aerospace Engineering;
J. Crayton Pruitt Family
Department of Biomedical Engineering;
Institute for Cell Engineering and
Regenerative Medicine,
University of Florida,
Gainesville, FL 32611

W. Gregory Sawyer

Department of Mechanical and
Aerospace Engineering;
Department of Materials
Science and Engineering,
University of Florida,
Gainesville, FL 32611
e-mail: wgsawyer@ufl.edu

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received August 13, 2015; final manuscript received November 9, 2015; published online July 26, 2016. Assoc. Editor: George K. Nikas.

J. Tribol 138(4), 042103 (Jul 26, 2016) (3 pages) Paper No: TRIB-15-1294; doi: 10.1115/1.4032890 History: Received August 13, 2015; Revised November 09, 2015

Gemini hydrogels have repeatedly produced low friction under conditions generally not thought to be favorable to superlubricity: low sliding speeds, low contact pressures, macroscopic contact areas, and room temperature aqueous environments. A proposed explanation for this unique behavior is that thermal fluctuations at the interface are sufficient to separate the surfaces, with solvent (water) shearing in this region being the main source of dissipation. In this paper, we demonstrate that very soft and correspondingly large mesh size Gemini hydrogels show superlubricity with the lowest measured friction coefficient being μ = 0.0013 ± 0.0006.

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Copyright © 2016 by ASME
Topics: Friction , Hydrogels
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Grahic Jump Location
Fig. 2

(a) A representative friction loop from the Gemini hydrogel sliding interface (probe radius of curvature ∼ 2 mm and countersample thickness ∼ 5 mm) subjected to a normal load of 500 μN, stroke length of 800 μm, and sliding speed of 200 μm/s. The black data in the free sliding region were used to calculate the average friction coefficients and associated uncertainties. (b) Friction coefficients from Fig. 1 were plotted together with those in this paper, and the relation in Urueña et al. was used to calculate the mesh size for the two measurements shown [22]. (c) The friction coefficient loop shows a low contact stiffness [29,30] as the probe moved from the reversal locations (1) to overcome breakloose friction [31] and eventually left the mutual overlap criterion [32] (2 and 3) to achieve superlubricity for the duration of sliding. (d) Friction coefficients of μ = 0.001–0.002 are among the lowest values reported in the book Superlubricity [2].

Grahic Jump Location
Fig. 1

Friction coefficients decreasing with increasing mesh size as 1/ξ was reported in Urueña et al. and predicted superlubricity for ξ > 5 nm [22]. All the reported values were below μ < 0.05, with the lowest value μ ∼0.005 obtained for a mesh size of ξ ∼ 10 nm. At this order of magnitude and below, the term “superlubricity” is used to describe such friction behavior [2]. Adapted from Ref. [22].

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