Session K52: Extreme Deformation of Polymers and Soft Matter I: Cavitation and Fracture

High Strain Rate, High Pressure Behavior of Polyurea

Elastomeric polyurea is an important polymer for impact mitigation, but viscoelasticity, nonlinearity, pressure-effects, and the onset of the glass transition complicate analyses of experimental measurements. Ballistic impact is characterized by high pressures and high strain rates; the effects of these on material properties and their relationship to ballistic performance is not well understood. The glass transition plays an important role in rate-dependent phenomena, therefore we measured the glass transition temperature, T_g, up to pressures of 6 GPa in a diamond anvil cell. Additionally, measurements of the ballistic performance as a function of temperature are presented in order to understand polyurea’s various dissipative mechanisms, and show results from high strain rate experiments utilizing infrared thermography so that we can answer the question: where does the energy go during deformation?   

Ultrasoft Fracture Energies via Cavitation Rheology

The developing technique of cavitation rheology (CR) probes hydrostatic failure via void pressurization. Hydrostatic failure is hypothesized as a primary injury mechanism in blast and blunt force trauma and can be a side-effect of ultrasound therapies. More fundamentally, no standard method has been developed for the determination of failure properties in ultrasoft polymer networks. Past CR measurements of synthetic, polymeric materials at length scales from mm’s to μm’s have been found to correlate with elastic modulus and fracture energy. This technique is performed via pressurization of fluid within a needle that is embedded within a material. Crack shape is quantified using micro-computed tomography and shown to transition from being roughly penny-shaped, to lobed, to spherical as a function of microstructure (swelling and cross-linking). Our morphology map predicts crack shape as a function of microstructure. By combining scaling analysis with cavitation rheology boundary conditions, we generate quantitative fracture energies for ultrasoft polymer networks that are consistent with fracture energies obtained from the pure shear test geometry.   

Cavitation in soft materials

Cavitation in soft materials have been intensively studied both theoretically and experimentally in the past. In the presentation, we would like to discuss some of our recent progresses on cavitation in soft materials. By introducing a small ring-crack on the wall of a cavity in a soft material, we investigated the cavitation-induced fracture in soft materials. We found that depending on the loading conditions and material properties, cavitation in soft material may lead to steady or unsteady growth of a crack. Our results will be very useful for understanding the entire cavitating process in soft materials. Considering most soft biological tissues and some engineering soft materials are not homogenous, we also studied cavitation in inhomogeneous soft materials. It is found that dramatically different from cavitation in homogenous material, the pressure vs. cavity volume curve can become nonmonotonic for inhomogeneous materials, which may result in snap-through instabilities.
  

High Strain-rate Soft Material Characterization via Inertial Cavitation

Mechanical characterization of soft materials at high strain-rates is challenging due to their high compliance, slow wave speeds, and non-linear viscoelasticity; however, knowledge of their material behavior is paramount across a spectrum of biological and engineering applications. To address this significant experimental hurdle and accurately measure the viscoelastic properties of soft materials at high strain-rates, we present a minimally invasive, local, 3D microrheology technique based on inertial microcavitation. By combining high-speed time-lapse imaging with a theoretical cavitation-modeling framework, we demonstrate that this technique has the capability of accurately determining the viscoelastic material properties of soft matter as compliant as a few kilopascals at high strain-rates. Given its straightforward implementation into most current microscopy setups, we anticipate that this technique may be easily adopted by researchers interested in characterizing soft material properties at high loading rates including hydrogels, tissues, and various polymeric specimens.   

Laser-induced Cavitation for Understanding High-strain-rate Mechanical Behavior of Soft Materials

High-strain-rate mechanical properties of weakly cross-linked swollen polymeric system are relevant in understanding the dynamics and damage mechanisms of biological tissues under high-rate mechanical stimuli. We create laser-induced cavitation within synthetic soft materials and animal tissues using ablation seeds, which enable more consistent ablation. By absorbing a nanosecond infrared laser pulse, the ablation seed is vaporized and produces a rapidly expanding cavity. The expansion dynamics of the laser-induced cavity is observed using ultrafast laser pulses. In addition to the high-strain-rate cavitation behavior, structural features of the specimens before and after cavitation are also compared in order to correlate the real-time cavitation dynamics and structural characteristics of the soft materials. Our laser-induced cavitation approach can provide quantitative information and insight into the mechanisms that are important in the response of the soft materials under extreme loading conditions.   

Fracture and healing of elastomers: Experiments, Theory, and numerical implementation

The recent numerical analysis by Lefèvre et al. (Int. J. Frac., 2015) and experiments by Poulain et al. (Int. J. Frac., 2017) have provided a thorough qualitative picture of the phenomena of nucleation and the ensuing growth of internal cavities/cracks in elastomers. Inter alia, this qualitative picture has established that the nucleation of internal cavities/cracks, popularly called cavitation, is fundamentally a by-product of fracture and not solely of elasticity as popularly thought. Also the experiments have revealed that internally nucleated cracks in conventional elastomers may fully heal.
In this talk, I will review, describe, and explain the above recent experiments in light of a new macroscopic theory that views elastomers as solids capable to undergo finite elastic deformations and also to phase transition to another solid of vanishingly small stiffness: the forward phase transition models the nucleation and propagation of fracture while the reverse phase transition models the possible healing. Further, guided by the experiments, the phase transition is taken to be driven by the competition between a combination of strain energy and hydrostatic stress concentration in the bulk and surface energy on the created/healed new surfaces in the given elastomer.   

Molecular Dynamics Simulation of Cavitation in Network Polymer Gels

Cavitation, the sudden formation and expansion of a void, causes both reversible and irreversible changes in soft materials. While extensively studied in simple fluids, there is little insight into cavitation in soft solids which contain both structural components and a fluid phase. Crosslinked polymer networks swollen in solvent, such as hydrogels, have gained growing attention due to properties that mimic natural materials and soft tissues. As network structure is intimately related to material properties, it is important to gain insight into the dynamic behavior of the network during mechanical deformation. Coarse-grained molecular dynamics is used to study the response of swollen polymer networks to isotropic dilation, and we examine the connection between local structure and cavity formation in gels. We use tensile deformation to induce cavitation in both model perfect diamond networks and defect-containing networks as a function of the polymer concentration and strand length. We find that the local structure is highly correlated with cavity formation, and surprisingly we find that the cavities tend to form in specific locations in the network and are not randomly distributed. Finally, we conclude by characterizing the structural features that promote cavitation.   

Failure Behavior of Alginate Hydrogels

Polysaccharide networks are widely used in biomedical and food applications. In this presentation, results on alginate hydrogels, a model polysaccharide network, will be presented. Alginate gels with two different types of crosslinking, ionic and covalent, are considered. Calcium salts have been used to obtain ionically crosslinked gels. Diamines and amine functionalized carbon nanodots have been used to synthesize covalently crosslinked gels. Small angle x-ray scattering captures the structural differences in ionic and covalently crosslinked gels. These gels display strain stiffening behavior in shear-rheology. A custom built cavitation rheology technique was used to capture the failure behavior of these gels. Here, at the critical pressure, instead of a spherical cavity growth at the needle tip, fracture in gel has been observed. The critical pressure changes with the type of crosslinking and as a function of crosslinker concentration. The fracture propagation has been found to be different in the ionic and covalently crosslinked gels.   

Cavitation Rheology of Biological Materials

The overall physical properties of complex soft materials and tissues emerge in a complex manner from the properties of the component materials across multiple length scales ranging from nanometres to millimetres. Cavitation rheology (CR) has been used to characterise the mechanical properties of strong materials made from proteins and also cells within cell spheroids (a tumour model). I will discuss the formation and characterisation using CR of protein bigels which are stretchable hydrogels with large fracture energies formed from two interpenetrating but discrete protein networks [1]. I will then discuss how CR can be used to measure the interfacial properties and the elastic modulus of spheroids formed from HEK cells. By comparing the work of bubble formation with deformation of the cell spheroid at different length scales, the cortical tension for cells within a cell spheroid has been estimated [2].   

Residual Stress Effects on Needle-Induced Cavitation

Needle-induced cavitation (NIC) measures the local stiffness of soft solids through pressurized fluid injection and can be performed in vivo. The critical pressure observed in NIC has been connected to the elasticity, surface tension, and fracture properties of the gel. Current elasticity models of NIC assume a fully relaxed initial deformation state, but deep penetration and puncture by the needle prior to injection can present significant residual stresses that have the potential to alter both the critical pressure and deformation mechanism observed. The effect of puncture process mechanics, and resulting residual stresses, was experimentally measured by monitoring the force on the needle during the cavitation process. Residual stress was found to alter the observed critical pressure, and this effect was interpreted through a simple, analytical model of the deep insertion process. Understanding the impact of the needle insertion process on NIC will enable confident measurement of local mechanical properties in soft gels and tissues.   

A Device for High-Strain-Rate Cavitation

Cavitation events within and near soft solids can arise from shock loading, phase change, and acoustic waves; all involve high-rate failure at the surface of the deforming solid. In exploring these, the rate of loading and total energy input can be either challenging or impossible to control independently, further complicating analysis of high rate elastic cavitation. Recently, a technique known as cavitation rheology has been used to understand both elastic and failure responses of soft solids through the pressurization of a cavity, at a controlled rate. However, the loading rates allowed by a typical instrument cannot probe the high rates typical of the extreme loads listed above. To address this, we have developed a device that capable of high-strain-rate cavitation. The cavities that form mimic temporary cavities observed in ballistic impact. An experiment is performed by injecting high pressure air, moving at the acoustic limit and controlled via a piezo-actuated valve, into a soft material. Energy input from a single injection is controlled via the combination of the reservoir temperature and pressure and the actuation time. Using a range of pressures (3-30 MPa) and needle diameters (100-130 μm) we can impart energy densities on time scales relevant to ballistic impacts.   

Deformation and Puncture of Synthetic Tissue Surrogates under Impact Loading

Penetration resistant textiles are used to protect vital organs from perforation and blunt trauma during a medium to high velocity event. Current standards measure the ability of protective to resist penetration from a standard threat propelled at impact energies up to 75 J. Fabric penetration, by the threat, represents a critical injury for the wearer. In order to better mimic real world physics, soft elastomers or foams are employed to replicate the compliance and viscoelasticity of tissue. In the case of a non-penetrating impact, little information may be gleaned about secondary injuries from blunt trauma. Better dynamic measurements on deformation and fracture properties of soft tissue surrogate materials under high energy, medium velocity indentation may lead to test methods for assessing non-critical injuries. In this work, an instrumented impactor is used to measure acceleration and force during impact of silicone and foam tissue surrogates at velocities up to 7 m/s and energies up to 90 J. High speed videography is utilized to quantify local strains, surface deflections, and initiation of fracture during impact. The effect of surrogate network structure and fracture toughness on dynamic response and the potential for assessment of soft tissue damage is presented.   

Large strain rheology plays a key role in the peeling of a soft adhesive

Modeling the exceptional adherence properties of Pressure Sensitive Adhesives (PSA) from the material properties of the adhesive itself and from surface interactions remains a challenge. The presence of a complex debonding region where the adhesive undergoes cavitation and the very large strain of a spontaneously formed fibrillar network has defied many modeling attempts over the past 70 years. We present here the important insights made possible by the development of an instrumented peeling technique [Int. J. Fracture, 204, 175 (2017)], based on high resolution imaging of the shape of the debonding region during peeling. The fine characterization of the response of this fibrillar region allows a quantitative linking of the adherence performances in peeling with the extensional rheology of the adhesive, in agreement with a recently established model [Soft Matter, 11, 3480, (2015)]. This investigation opens the way towards the modeling of the fibril debonding criterion, which is the main missing link towards the prediction of the adherence properties.