Perforation of thin-membrane-covered soft matter induced by cavitation-initiated extreme mechanics

Traumatic events can initiate cavitation within the cerebrospinal fluid surrounding the human brain. While the brain is soft [Young’s modulus <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mi>E</a:mi><a:mo>∼</a:mo><a:mi>O</a:mi><a:mo stretchy="false">(</a:mo><a:mn>1</a:mn><a:mo stretchy="false">)</a:mo></a:math> kPa], a thin membrane named cranial pia mater [<f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><f:mi>E</f:mi><f:mo>∼</f:mo><f:mi>O</f:mi><f:mo stretchy="false">(</f:mo><f:mn>1</f:mn><f:mo stretchy="false">)</f:mo></f:math> MPa] separates the cerebral cortex from the liquid. Here, we reveal the effect of this composite boundary on the damage potential of cavitation through well-controlled experiments. Therefore, a soft hydrogel is covered by a thin biomembrane and is exposed to cavitation bubbles created at various distances with a pulsed laser. High-speed imaging reveals a perforation of the membrane and penetration of a liquid jet deep into the composite structure from a single event, and its strong dependence on the stand-off distance of the bubble to the boundary. When the stand-off distance is around 0.65, needlelike liquid jets in excess of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><k:mn>1000</k:mn></k:math> m/s can be formed that pass through the bubble without atomization. The later visible shock-wave emission is related to the phenomenon of shock-wave self-focusing.