Prefracture Instabilities Govern Generation of Self-Affine Surfaces in Tearing of Soft Viscoelastic Elastomeric Sheets

Abstract

We present an experimental study on fracture behavior of soft viscoelastic network solid films of polydimethylsiloxane (PDMS) and morphology of the fracture-generated surfaces. The most interesting behavior is the generation of fractal patterns when the storage and loss moduli are comparable. We find the formation of isolated defects and cracks ahead of the fracture-front. The observed morphology of the torn surfaces is correlated to the prefracture behavior by studying the crack propagation and coalescence. Tearing of PDMS containing 2% crosslinking agent with a terminal elastic modulus, G′, of 10 kPa, shows distinct crack patterns at different tearing speeds. Slower tearing leads to the formation of dendritic patterns of propagating fingers, whereas at the higher deformation-rates, more frequent nucleation of bubble like defects occurs. The propagating fingers develop undulations along its sides. The effects of tearing speed on the length scale of the undulations and the bubbles are studied. The formation of undulations can be explained as a result of Saffman−Taylor instability with a modified effective surface tension which accounts for the dissipative energy loss. We also study the rate of formation of the defects, which is shown to be an activated process in that they form beyond a threshold value of the local stress. ■ INTRODUCTIONMaterials undergo cohesive failure by crack formation and propagation under excessive stress. Study of precrack events and morphology of cracks provides vital clues regarding the nature of the texture generated on the surfaces formed by the fracture and can also be a diagnostic tool for characterizing the failure mechanisms and the properties of the failed material. Materials with different mechanical properties exhibit different morphologies on their fractured surfaces. Ductile fracture in metals 1−3 is accompanied by plastic deformation prior to the fracture where the microvoids coalesce leading to a dimpled, irregular, and rough morphology commonly known as the cup-and-cone morphology. Brittle fracture in metals involves cleavage along crystallographic planes resulting into relatively smoother or faceted surfaces. 4,5 Fracture in polymeric materials has particularly drawn considerable scientific interest because of the complexity of the phenomena and richness of the resulting morphologies. In elastomers such as vulcanized polybutadiene, 6−8 vertical steps separating smooth regions form the characteristic features, whereas in the glassy polymers such as polystyrene, poly-(methyl methacrylate), and polycarbonate, periodic micro/ nano structured gratings are formed on the fractured surfaces. 9 Spatiotemporal patterns have been observed in peeling of adhesive tapes.10 While adhesive interfacial failure of elastomeric surfaces have been extensively studied, 11−14 patterns formed by tearing of elastomeric sheets have not been explored. Previous studies on interfacial debonding have shown both bulk and interfacial fingering instabilities and cavitation during separati...

Making Nonsticky Surfaces of Sticky Materials: Self-Organized Microtexturing of Viscoelastic Elastomeric Layers by Tearing

“…RMS roughness is calculated as the root mean square of a surface measured microscopic peaks and valleys. RMS roughness was obtained from Here, Z a is the average of the Z (height) values within the given area, Z i is the Z value at a given point, and N is the number of sampled points within the given area. Figure shows the roughness profiles at various tearing speeds for 20 and 40 μm film thicknesses (half of the total original thickness).…”

“…The detailed characterization of the fractured surfaces and the prefacture instabilities leading to the final petal-like structures is presented by Patil et al Morphology of the self-affine surfaces of the torn viscoelastic PDMS layers was characterized by power spectral density to access their fractal dimension. The fractal dimension of the surface was found close to 1.5, independent of the tearing speed (in the range of 1–500 μm/s).…”

One-Step Fabrication of Microchannels Lined with a Metal Oxide Coating

“…The deposited cuprous oxide layer on copper substrate (in forms of a thin copper sheet and a thin copper wire) was dried and embedded into commercially available 10% cross-link PDMS (Sylgard 184, Dow corning). The elastomer and cross-linking agents were mixed in a ratio of 100:10 w/w (or 10% cross-linked PDMS) separately in a beaker and degassed under vacuum in order to remove dissolved air from the PDMS solution. ,− First, the 10% cross-linked PDMS solution was cast between two clean glass plates with cuprous oxide coated metal wire and sheet embedded (Figure (a)). Complete block of cast PDMS with electrodeposited metal wire and metallic sheet embedded in it was kept for curing for 12 h at 80 °C.…”

Embedded macroporous elastomers by hydrostatic fracturing for flexible strain‐sensor applications

“…Relative importance of these two effects plays the governing role in determining the size and morphology of the fracture‐generated surfaces in viscoelastic materials. In an earlier study, similar aspects were investigated in surfaces generated by tearing or enforcing cohesive failure in viscoelastic elastomers. In Ref.…”

“…In Ref. , we studied cohesive failure under plane stress conditions generated by distributed extensional loading. In this article, we adopt a complementary approach and study fractures in soft viscoelastic elastomers formed under point loading that generates three‐dimensional hydrostatic pressure.…”