Transient and permanent cavitation in collagen gels by laser-induced thermoelastic pressure waves

Laser-induced cavitation inside model objects - gels with various collagen concentrations (10 - 20%) was studied. Cavitation was produced by thermoelastic pressure waves generated under stress-confined conditions. Gel slabs were irradiated in air by nanosecond laser pulses from both sides. Under these conditions bipolar pressure pulses with compressional and tensile phases were generated on each side of the gel slabs. Due to interaction of these pressure pulses transient compression and tension with the amplitude two times higher are produced in the middle of the gel slabs. The tensile phase can induced cavitation. Two types of cavitation were found: transient (with bubble collapse) and permanent leading to formation of stable bubbles. To measure threshold of transient and permanent cavitation two methods were proposed. Transient cavitation was detected by scattering of a He-Ne probe beam from the cavitation bubbles and observed only for gels with low collagen concentration. Transient cavitation threshold for the gel with 10%-concentration amounts 28 bar. Transient cavitation lasts for approximately 3 μs (approximately 1 μs - growth for cavitation bubbles and approximately 2 μs - collapse). Permanent cavitation threshold and total volume of permanent bubbles were studied by optical microscopy. Permanent cavitation can be divided into three types: cavitation with the following collapse accompanied by rupture and formation of small residual cavitation bubbles for gels with low (approximately 10%) collagen concentration; cavitation without collapse and with coalescence resulting in large bubble formation for gels with approximately 14% collagen concentration; cavitation without coalescence for gels with high approximately 20% collagen concentration.