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.

References

References
1.
Hughes
,
B. D.
, and
White
,
L. R.
,
1979
, “
‘Soft’ Contact Problems in Linear Elasticity
,”
Q. J. Mech. Appl. Math.
,
32
(
4
), pp.
445
471
.
2.
Kao
,
I.
, and
Yang
,
F.
,
2004
, “
Stiffness and Contact Mechanics for Soft Fingers in Grasping and Manipulation
,”
IEEE Trans. Rob. Autom.
,
20
(
1
), pp.
132
135
.
3.
Hwang
,
S.-W.
,
Lee
,
C. H.
,
Cheng
,
H.
,
Jeong
,
J.-W.
,
Kang
,
S.-K.
,
Kim
,
J.-H.
,
Shin
,
J.
,
Yang
,
J.
,
Liu
,
Z.
,
Ameer
,
G. A.
,
Huang
,
Y.
, and
Rogers
,
J. A.
,
2015
, “
Biodegradable Elastomers and Silicon Nanomembranes/Nanoribbons for Stretchable, Transient Electronics, and Biosensors
,”
Nano Lett.
,
15
(
5
), pp.
2801
2808
.
4.
Barquins
,
M.
, and
Courtel
,
R.
,
1975
, “
Rubber Friction and the Rheology of Viscoelastic Contact
,”
Wear
,
32
(
2
), pp.
133
150
.
5.
Krick
,
B. A.
,
Vail
,
J. R.
,
Persson
,
B. N.
, and
Sawyer
,
W. G.
,
2012
, “
Optical In Situ Micro Tribometer for Analysis of Real Contact Area for Contact Mechanics, Adhesion, and Sliding Experiments
,”
Tribol. Lett.
,
45
(
1
), pp.
185
194
.
6.
McCutchen
,
C. W.
,
1964
, “
Optical Systems for Observing Surface Topography by Frustrated Total Internal Reflection and by Interference
,”
Rev. Sci. Instrum.
,
35
(
10
), pp.
1340
1345
.
7.
Ovcharenko
,
A.
,
Halperin
,
G.
,
Etsion
,
I.
, and
Varenberg
,
M.
,
2006
, “
A Novel Test Rig for In Situ and Real Time Optical Measurement of the Contact Area Evolution During Pre-Sliding of a Spherical Contact
,”
Tribol. Lett.
,
23
(
1
), pp.
55
63
.
8.
Erath
,
J.
,
Schmidt
,
S.
, and
Fery
,
A.
,
2010
, “
Characterization of Adhesion Phenomena and Contact of Surfaces by Soft Colloidal Probe AFM
,”
Soft Matter
,
6
(
7
), pp.
1432
1437
.
9.
Mow
,
M. C.
, and
Ling
,
F. F.
,
1969
, “
On Weeping Lubrication Theory
,”
Z. Angew. Math. Phys. ZAMP
,
20
(
2
), pp.
156
166
.
10.
Ling
,
F. F.
,
1974
, “
A New Model of Articular Cartilage in Human Joints
,”
ASME J. Tribol.
,
96
(
3
), pp.
449
454
.
11.
Kirschner
,
C. M.
, and
Anseth
,
K. S.
,
2013
, “
Hydrogels in Healthcare: From Static to Dynamic Material Microenvironments
,”
Acta Mater.
,
61
(
3
), pp.
931
944
.
12.
Peppas
,
N. A.
, and
Sahlin
,
J. J.
,
1996
, “
Hydrogels as Mucoadhesive and Bioadhesive Materials: A Review
,”
Biomaterials
,
17
(
16
), pp.
1553
1561
.
13.
Chan
,
E. P.
,
Hu
,
Y.
,
Johnson
,
P. M.
,
Suo
,
Z.
, and
Stafford
,
C. M.
,
2012
, “
Spherical Indentation Testing of Poroelastic Relaxations in Thin Hydrogel Layers
,”
Soft Matter
,
8
(
5
), pp.
1492
1498
.
14.
Chen
,
X.
,
Dunn
,
A. C.
,
Sawyer
,
W. G.
, and
Sarntinoranont
,
M.
,
2007
, “
A Biphasic Model for Micro-Indentation of a Hydrogel-Based Contact Lens
,”
ASME J. Biomech. Eng.
,
129
(
2
), pp.
156
163
.
15.
Long
,
R.
,
Hall
,
M. S.
,
Wu
,
M.
, and
Hui
,
C.
,
2011
, “
Effects of Gel Thickness on Microscopic Indentation Measurements of Gel Modulus
,”
Biophys. J.
,
101
(
3
), pp.
643
650
.
16.
Dimitriadis
,
E. K.
,
Horkay
,
F.
,
Maresca
,
J.
,
Kachar
,
B.
, and
Chadwick
,
R. S.
,
2002
, “
Determination of Elastic Moduli of Thin Layers of Soft Material Using the Atomic Force Microscope
,”
Biophys. J.
,
82
(
5
), pp.
2798
2810
.
17.
Hall
,
M. S.
,
Long
,
R.
,
Hui
,
C.
, and
Wu
,
M.
,
2012
, “
Mapping Three-Dimensional Stress and Strain Fields Within a Soft Hydrogel Using a Fluorescence Microscope
,”
Biophys. J.
,
102
(
10
), pp.
2241
2250
.
18.
Dintwa
,
E.
,
Tijskens
,
E.
, and
Ramon
,
H.
,
2008
, “
On the Accuracy of the Hertz Model to Describe the Normal Contact of Soft Elastic Spheres
,”
Granular Matter
,
10
(
3
), pp.
209
221
.
19.
Sawyer
,
W. G.
,
Argibay
,
N.
,
Burris
,
D. L.
, and
Krick
,
B. A.
,
2014
, “
Mechanistic Studies in Friction and Wear of Bulk Materials
,”
Ann. Rev. Mater. Res.
,
44
(
1
), pp.
395
427
.
20.
Uruena
,
J. M.
,
Pitenis
,
A. A.
,
Nixon
,
R. M.
,
Schulze
,
K. D.
,
Angelini
,
T. E.
, and
Sawyer
,
W. G.
,
2015
, “
Mesh Size Control of Polymer Fluctuation Lubrication in Gemini Hydrogels
,”
Biotribology
,
1–2
, pp.
24
29
.
21.
Lin
,
W.
,
Shull
,
K. R.
,
Hui
,
C.
, and
Lin
,
Y.
,
2007
, “
Contact Measurement of Internal Fluid Flow Within Poly (n-Isopropylacrylamide) Gels
,”
J. Chem. Phys.
,
127
(
2
), p.
094906
.
22.
White
,
M. L.
, and
Dorion
,
G. H.
,
1961
, “
Diffusion in a Crosslinked Acrylamide Polymer Gel
,”
J. Polym. Sci.
,
55
(
162
), pp.
731
740
.
23.
Johnson
,
K. L.
,
1987
,
Contact Mechanics
,
Cambridge University Press
,
Cambridge, UK
.
24.
Hu
,
Y.
,
Chan
,
E. P.
,
Vlassak
,
J. J.
, and
Suo
,
Z.
,
2011
, “
Poroelastic Relaxation Indentation of Thin Layers of Gels
,”
J. Appl. Phys.
,
110
(
8
), p.
086103
.
25.
Boudou
,
T.
,
Ohayon
,
J.
,
Picart
,
C.
,
Pettigrew
,
R. I.
, and
Tracqui
,
P.
,
2009
, “
Nonlinear Elastic Properties of Polyacrylamide Gels: Implications for Quantification of Cellular Forces
,”
Biorheology
,
46
(
3
), pp.
191
205
.
26.
Gong
,
J. P.
,
Katsuyama
,
Y.
,
Kurokawa
,
T.
, and
Osada
,
Y.
,
2003
, “
Double-Network Hydrogels With Extremely High Mechanical Strength
,”
Adv. Mater.
,
15
(
14
), pp.
1155
1158
.
27.
Witten
,
T. A.
, and
Pincus
,
P. A.
,
2004
,
Structured fluids: Polymers, Colloids, Surfactants
,
Oxford University Press
, New York.
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