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Research Papers: Contact Mechanics

# Real Area of Contact in a Soft Transparent Interface by Particle Exclusion Microscopy

[+] Author and Article Information
Kyle D. Schulze, Alex I. Bennett, Samantha Marshall, Kyle G. Rowe

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

Alison C. Dunn

Department of Mechanical
Science and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801
e-mail: acd@illinois.edu

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received July 1, 2015; final manuscript received November 16, 2015; published online July 20, 2016. Assoc. Editor: Robert L. Jackson.

J. Tribol 138(4), 041404 (Jul 20, 2016) (6 pages) Paper No: TRIB-15-1234; doi: 10.1115/1.4032822 History: Received July 01, 2015; Revised November 16, 2015

## Abstract

Soft matter mechanics are characterized by high strains and time-dependent elastic properties, which complicate contact mechanics for emerging applications in biomedical surfaces and flexible electronics. In addition, hydrated soft matter precludes using interferometry to observe real areas of contact. In this work, we present a method for measuring the real area of contact in a soft, hydrated, and transparent interface by excluding colloidal particles from the contact region. We confirm the technique by presenting a Hertz-like quasi-static indentation (loading time > 1.4 hrs) by a polyacrylamide probe into a stiff flat surface in a submerged environment. The real contact area and width were calculated from in situ images of the interface processed to reduce image noise and thresholded to define the perimeter of contact. This simple technique of in situ particle exclusion microscopy (PEM) may be widely applicable for determining real areas of contact of soft, transparent interfaces.

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## Figures

Fig. 1

(a) PEM is performed by pushing an instrumented soft probe against a flat dish in a suspension of small particles < 500 nm in diameter and simultaneously imaging the contact using an inverted objective. (b) The particles are squeezed out and excluded from the intimate contact between the smooth probe and smooth flat. (c) (Left): The recorded image with distinct dark outer region and lighter center. (Right): The image thresholded with regard to this boundary to establish contact as a white center on a black background. (d) As contact area increases, suspended particles flow away from contact; the accuracy of measurement increases as the contact area increases.

Fig. 2

(a) The lengths of slices along one axis of contact (shown as horizontal) 10 pixels wide and centered about the approximate centroid of contact were averaged to determine the contact diameter 2 a for each image. (b) A kymograph concisely presents the average contact diameter over time for five successive indentations using the same probe.

Fig. 3

(a) A vertical piezoelectric stage pushed the probe into the flat according to a linear profile. (b) The force of indentation increased from ∼200 μN up to 2000 μN over 5000 s, or ∼0.36 μN/s. (c) The contact width correspondingly increased to over 1000 μm at the maximum load. (d) Sample images at three intermittent points showed increasing area with increasing force.

Fig. 4

Particle size determines the magnitude of error between the real and measured radii of contact (top). Assuming a spherical particle is excluded from contact but remains tangent to the indenting spherical probe and flat countersurface, the geometric difference between the real radius of contact and the measured radius of contact can be calculated and plotted for lines of constant particle radius (bottom). The dashed region indicates the upper bound of error experimental conditions for this work based on reported particle size and probe size.

Fig. 5

At loads greater than ∼300 μN, this force–deflection curve appears to conform to the power-law relationship predicted from Hertz contact equations

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