Competition between fragmentation and defragmentation of сarbon black nanoparticle agglomerates under a degenerate optical microcavitation mode

This paper studies the nonlinear transformation process of carbon black nanoparticle agglomerates in water under nanosecond near-infrared laser irradiation. An experimental and theoretical relationship between the sizes of a microbubble and a carbon microparticle has been obtained. A thermophysical calculation based on an analytical solution of the heat conduction equation for a homogeneous sphere and infinite medium possess different thermal properties, allowed analyzing the dynamics of a carbon microparticle cooling in the degenerate optical microcavitation modes (long-lived microbubbles) and tracing the trend of changes in the accumulated temperature depending on laser parameters. A region of parameters (pulse number, repetition rate) in which the transition from the fragmentation to defragmentation of the agglomerates occurs has been experimentally found. Differentiation of these processes contributed to the definition of the role of degenerate optical microcavitation in combination with accumulative effects and the detailing of their mechanisms. A fragmentation mechanism has been established that takes into account degenerate optical microcavitation. A mechanism for defragmentation of carbon black nanoparticle agglomerates has been proposed based on the creation of prerequisites for water transition into the state of a supercritical fluid accompanied by partial agglomerate dissolution in laser-induced long-lived microbubbles at the cooling stage and on the decrease of agglomerate maximum heating temperature because of increased light scattering by long-lived microbubbles. The regularities studied are important for the development of laser technologies for destruction of carbon black nanoparticle agglomerates in liquid media, including biological ones.