The inherently fragile nature of ultrathin polymer films presents difficulties to the measurement of their mechanical properties, which are of interest in packaging, electronics, separations, and other manufacturing fields. More fundamentally, the direct measurement of ultrathin film mechanical properties is necessary for understanding changes in intrinsic material properties at reduced size scales, for example, when the film thickness alters the equilibrium configuration of the polymer chains. We introduce a method for ultrathin film tensile testing that stretches a twodimensionally macroscopic, yet nanoscopically thin, polymer film on the surface of water. For polystyrene films, we observe a precipitous decrease in mechanical properties (Young's modulus, strain at failure, and nominal stress at failure) for film thicknesses down to 15 nm, less than the characteristic size of an individual polymer chain, yielding new insights into the changes in polymer chain entanglements in confined states.

Typical TEM images of CNTs extracted from an undeformed pillar. In situ videos: (included)PillarCompression_25xspeed.mpg PillarCompression_25xspeed.avi A pillar compression performed at a strain rate of 0.001 s -1 , then sped up to 25× the original rate for easier viewing.PillarCompression_25xspeed_data.mpg PillarCompression_25xspeed_data.aviStress-strain data gathered simultaneously with the pillar compression shown in 'PillarCompression_25xspeed.mpg'. Both videos may be played side-by-side to correlate the buckling events and stress-strain humps.

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Gels and other soft elastic networks are a ubiquitous and important class of materials whose unique properties enable special behavior, but generally elude characterization due to the inherent difficulty in manipulating them. An example of such behavior is the stability of gels to large local deformations on their surface. This paper analyzes puncture of model soft materials with particular focus on the force response to deep indentation and the critical load for material failure. Large-strain behavior during deep indentation is described with a neo-hookean contact model. A fracture process zone model is applied to the critical load for puncture. It is found that the indenter geometry influences the size of the fracture process zone, resulting in two distinct failure regimes: stress-limited and energy-limited. The methods outlined in this paper provide a simple means for measuring Young's modulus, E, as well as the material's maximum cohesive stress, σ0, fracture energy, Γ0, and the intrinsic length scale linking the two, l0, all without requiring specialized sample preparation.

A generalized Rayleigh-Plesset-type bubble dynamics model with a damage mechanism is developed for cavitation and damage of soft materials by focused ultrasound bursts. This study is linked to recent experimental observations in tissue-mimicking polyacrylamide and agar gel phantoms subjected to bursts of a kind being considered specifically for lithotripsy. These show bubble activation at multiple sites during the initial pulses. More cavities appear continuously through the course of the observations, similar to what is deduced in pig kidney tissues in shock-wave lithotripsy. Two different material models are used to represent the distinct properties of the two gel materials. The polyacrylamide gel is represented with a neo-Hookean elastic model and damaged based upon a maximum-strain criterion; the agar gel is represented with a strain-hardening Fung model and damaged according to the strain-energy-based Griffith's fracture criterion. Estimates based upon independently determined elasticity and viscosity of the two gel materials suggest that bubble confinement should be sufficient to prevent damage in the gels, and presumably injury in some tissues. Damage accumulation is therefore proposed to occur via a material fatigue, which is shown to be consistent with observed delays in widespread cavitation activity.