Chapter 21 Assessment of Fluid Cavitation Threshold Using a Polymeric Split Hopkinson Bar-Confinement Chamber Apparatus Michael C. Bustamante and Duane S. Cronin Abstract Mild Traumatic Brain Injury (mTBI) has been associated with blast exposure resulting from the use of improvised explosive devices (IEDs) in recent and past military conflicts. Experimental and numerical models of head blast exposure have demonstrated the potential for high negative pressures occurring within the head at the contre-coup location relative to the blast exposure, and it has been hypothesized that this negative pressure could result in cavitation of Cerebrospinal Fluid (CSF) surrounding the brain, leading to brain tissue damage. The cavitation threshold of CSF, the effect of temperature, and the effect of impurities or dissolved gases are presently unknown. In this study, a novel Polymeric Split Hopkinson Pressure Bar and confinement chamber apparatus were used to generate loading in distilled water similar to the conditions in the vicinity of the CSF during blast exposure. Cavitation was identified using high-speed imaging of the event, and a validated numerical model of the apparatus was applied to determine the pressure in the fluid during the exposure. Increasing the water temperature resulted in a decrease in the 50% probability of cavitation from 21 ◦ C(−3320 kPa ± 3%) to 37 ◦ C (−3195 kPa ± 5%) in agreement with the theoretical values, but was not statistically significant. Importantly, the effect of water treatment had a significant effect on the cavitation pressure for water with wetting agent (−3320 kPa ± 3%), degassed water (−1369 kPa ± 16%) and untreated distilled water (−528 kPa ± 25%). Thus, reducing dissolved gases through degassing or the use of a wetting agent significantly increases the cavitation pressure and reduces the variability of the cavitation pressure threshold. Keywords Fluid cavitation · Polymeric Split Hopkinson Pressure Bar · Mild traumatic brain injury · Negative pressure 21.1 Introduction Mild Traumatic Brain Injury associated with blast exposure has been a prominent topic of study as a result of the increasing use of IEDs. Brain injury is categorized as mTBI when the individual experiences an alteration or loss of consciousness for up to 30 min [1–3]. There is currently no consensus on the mechanisms that can cause mTBI, with some suggesting: shearing damage of soft-tissue [4–7], distortion of brain cellular structures [5, 8–14], and intracranial fluid cavitation [5, 15–19]. With regard to cavitation, a number of studies have demonstrated that negative pressure can occur within the head at the contre-coup location relative to the blast exposure, as a result of pressure wave propagation initiated by a blast wave [16, 19–28]. There is a possibility of cavitation if the negative CSF pressure exceeds the tensile threshold, resulting in the sudden inception, growth, and collapse of a cavitation bubble. Theoretically, the implosive collapse of a near-vacuum bubble results in localized compressive pressures and temperatures several magnitudes above ambient pressure and temperature, respectively [29–31]. CSF cavitation in vivo has not been observed and the negative pressure threshold for cavitation is currently unknown. Several numerical studies investigating cavitation due to blast limit the minimum CSF pressure to −100 kPa or the equivalent of 1 atmosphere to simulate the occurrence of cavitation [16, 17, 25, 32]. The aim of this study was to determine the effects of fluid degassing treatment and temperature on the cavitation pressure threshold using distilled water. M. C. Bustamante () · D. S. Cronin Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada e-mail: mcbustam@uwaterloo.ca © The Society for Experimental Mechanics, Inc. 2019 M. Grady et al. (eds.), Mechanics of Biological Systems & Micro-and Nanomechanics, Volume 4, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-319-95062-4_21 95