Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99751

99751 - Soft Contact Mechanics of Gradient-Stiffness Surfaces Under Indentation Loading: Simulation and Experiment 

The contacting interfaces found in the body, such as cornea and articular cartilage, have a softer outer surface, which gradually stiffens into the depth. Similar gradient-stiffness surfaces of a chemically crosslinked hydrogel can be synthetically fabricated, which have high potential to mimic a robust, but extremely lubricious biological surface. When such gradient-stiffness surfaces undergo indentation loading, their contact mechanics largely depend on the properties of the outermost surface. Thus, the mechanical response of varying elasticity layers under indentation is a complex, cumulative response from the top surface to all the underlying sub-surfaces, which is difficult to completely understand within the framework of the existing contact mechanics theories.
Here, we have simulated a conventional quasistatic indentation experiment of an indenter pressed against a hydrogel surface which has specific gradient elastic layers. The depth of the gradient layer in the sample is varied as 0.1R, 0.27R, 0.74R, and 2R, where R is the probe radius. An axisymmetric model of conventional spherical indentation is created using finite elements (ABAQUS). To easily introduce depth-dependent elasticity as an input material property, we assign a pseudo-temperature-dependent modulus to node sets in layers into the depth. The validation of the simulated results is done by both micro- and nano-scale indentation experiments using microtribometer and colloidal probe Atomic Force Microscopy (AFM). For indentation experiments, the polyacrylamide hydrogel samples are made against different molding materials, such as glass and polystyrene (PS) to tune the depth of gradient layers of chemically equivalent hydrogels.
Our results show that an initial non-Hertzian force response was followed by a Hertzian force response when the indentation depth increases gradually. The stretch field reveals that the top gradient layers confine between the stiffer bulk and the hard probe and push the stretch field closer to the surface and radially outward. Therefore, the contact area is underestimated by the Hertz prediction, and consequently, the average contact pressure is lower. The indentation results for PS- and glass-molded samples support this transition of non-Hertzian to Hertzian force response at micro- and nano-scale experiments, respectively. However, the glass-molded sample at microscale indentation does not show any non-Hertzian initial force response due to having a much smaller depth of gradient layers compared to the probe radius (R=2mm), whereas the PS-molded sample at nanoscale indentation does not reach to Hertzian force response due to having a much larger depth of gradient layers compared to probe radius (R=5um).
Additionally, the adhesion of such gradient stiffness surfaces, especially at shallow indentation depths, is found to be significantly controlled by the outmost softer layers.
These findings will facilitate a better understanding of the force response of gradient stiff surfaces, found in many soft matters. In addition, it can provide useful guidelines to design hydrogels with a predetermined mechanical response for customized applications.

Presenting Author: Md Mahmudul Hasan University of Illinois at Urbana Champaign

Presenting Author Biography: Md Mahmudul HASAN is currently working as a Ph.D. researcher under Professor Alison C. Dunn in MechSE department at University of Illinois at Urbana Champaign (UIUC). His current research focuses on the area of micro/nano mechanics-driven tribology of soft biomimetic materials, especially hydrogel for emergent organ development and tissue engineering. Before joining UIUC, he was an ORCGS Ph.D. fellow at the University of Central Florida (UCF). Previously, Mr. Hasan completed his M.Sc. from Georgia Southern University (GSU), where he was working on nano-encapsulation of multiple cancer drugs for targeted, sustainable cancer therapy. As a favorite pass time, he likes to play cricket and to do camping.

Authors:

Md Mahmudul Hasan University of Illinois at Urbana Champaign
Alison C. Dunn University of Illinois at Urbana Champaign