Alexander Klein is a PhD student in the MESA+ research group Physics of Fluids. His supervisor is Detlef Lohse.
laser impact on flying drops
In the thesis, we study how a liquid drop responds to the impact of a laser pulse that induces a phase change concentrated both in time and space, i.e. localized to the drop surface and much shorter than the response of the drop. Combining high-speed with stroboscopic high-resolution imaging, the response is found to be violent: the drop is accelerated, strongly deforms, and eventually fragments. Shock waves, the ejection of matter, and even plasma generation can accompany this process. We resolve all relevant timescales of the drop response in a dedicated experimental setup to study the fluid dynamics in detail, complemented by boundary-integral simulations and theoretical modeling. Detailed understanding of the drop response upon laser impact is of key importance for the generation of extreme ultraviolet (EUV) light in the latest nanolithography machines for the fabrication of leading-edge semiconductor microchips.
The response of the drop to the laser impact can be summarized as a characteristic change of the liquid topology: the spherical drop deforms into a liquid sheet that expands radially under the influence of surface tension and finally fragments. We provide analytical descriptions for the kinematics of the deformation, which we validate extensively in one-to-one comparison to experimental results. In addition, we demonstrate that the interaction of a laser pulse with a liquid body can successfully be modeled by applying a recoil-pressure pulse to the liquid interface. The details of the laser-matter interaction are then described by an appropriate formulation of the pressure profile. The drop fragmentation is caused by Rayleigh--Taylor instabilities that destabilize the deforming liquid body, for which we derive scaling laws in terms of the characteristic time and wavenumber of breakup.
