Wilco Bouwhuis

Drop shaping by laser-pulse impact

We show how the deposition of laser energy induces propulsion and strong deformation of an absorbing liquid body. Combining high speed with stroboscopic imaging, we observe that a millimeter-sized dyed water drop hit by a millijoule nanosecond laser pulse propels forward at several meters per second and deforms until it eventually fragments. The drop motion results from the recoil momentum imparted at the drop surface by water vaporization. We measure the propulsion speed and the time-deformation law of the drop, complemented by boundary-integral simulations. The drop propulsion and shaping are explained in terms of the laser-pulse energy, the drop size, and the liquid properties. These findings are, for instance, crucial for the generation of extreme ultraviolet light in nanolithography machines.

Erosion evolution in mono-crystalline silicon surfaces caused by acoustic cavitation bubbles

The early stages (<180 min) of cavitation erosion of silicon surfaces were studied for three different crystallographic orientations. We introduce a quantity defined as the ratio of the relative eroded area to the number of pits, αp, to evaluate the evolution of erosion among the different substrates used. Different erosion evolution was observed for (100), (110), and (111) silicon surfaces when exposed to cavitation bubbles generated by an ultrasound signal of 191 kHz. (100) silicon substrates showed the most erosion damage, with an eroded area 2.5 times higher than the other two crystallographic orientation substrates after 180 min sonication. An apparent incubation period of 50 min was measured. The number of erosion pits increased monotonically for (110) and (111), but for (100) no increase was detected after 120 min. The collapse of a spherical bubble was simulated using an axisymmetry boundary integral method. The calculated velocity of the jet from the collapsing bubble was used to estimate the pressure P that is induced by the jet upon impact on the silicon substrate.

Universal mechanism for air entrainment during liquid impact

When a mm-sized liquid drop approaches a deep liquid pool, both the interface of the drop and the pool deform before the drop touches the pool. The build up of air pressure prior to coalescence is responsible for this deformation. Due to this deformation, air can be entrained at the bottom of the drop during the impact. We quantify the amount of entrained air numerically, using the Boundary Integral Method (BIM) for potential flow for the drop and the pool, coupled to viscous lubrication theory for the air film that has to be squeezed out during impact. We compare our results to various experimental data and find excellent agreement for the amount of air that is entrapped during impact onto a pool. Next, the impact of a rigid sphere onto a pool is numerically investigated and the air that is entrapped in this case also matches with available experimental data. In both cases of drop and sphere impact onto a pool the numerical air bubble volume V_b is found to be in agreement with the theoretical scaling V_b/V_{drop/sphere} ~ St^{-4/3}, where St is the Stokes number. This is the same scaling that has been found for drop impact onto a solid surface in previous research. This implies a universal mechanism for air entrainment for these different impact scenarios, which has been suggested in recent experimental work, but is now further elucidated with numerical results.

Drop deformation by laser-pulse impact

A free-falling absorbing liquid drop hit by a nanosecond laser-pulse experiences a strong recoil-pressure kick. As a consequence, the drop propels forward and deforms into a thin sheet which eventually fragments. We study how the drop deformation depends on the pulse shape and drop properties. We first derive the velocity field inside the drop on the timescale of the pressure pulse, when the drop is still spherical. This yields the kinetic-energy partition inside the drop, which precisely measures the deformation rate with respect to the propulsion rate, before surface tension comes into play. On the timescale where surface tension is important the drop has evolved into a thin sheet. Its expansion dynamics is described with a slender-slope model, which uses the impulsive energy-partition as an initial condition. Completed with boundary integral simulations, this two-stage model explains the entire drop dynamics and its dependance on the pulse shape: for a given propulsion, a tightly focused pulse results in a thin curved sheet which maximizes the lateral expansion, while a uniform illumination yields a smaller expansion but a flat symmetric sheet, in good agreement with experimental observations.

Laser impact on a drop

The energy deposition in a liquid drop on a nanosecond time scale by impact of a laser pulse can induce various reactions, such as vaporization or plasma generation. The response of the drop can be extremely violent: The drop gets strongly deformed and propelled forward at several m/s, and subsequently breaks up or even explodes. These effects are used in a controlled manner during the generation of extreme ultraviolet (EUV) light in nanolithography machines for the fabrication of leading-edge semiconductor microchips. Detailed understanding of the fundamentals of this process is of key importance in order to advance the latest lithography machines. - See more at: http://gfm.aps.org/meetings/dfd-2014/5408ec6e69702d07711b0200#sthash.26qTrDD5.dpuf

A multi-scale modeling approach for studying cortical lesions as a cause for epilepsy

Traumatic brain injury (TBI) may result in post-traumatic seizures and epilepsy. Approximately 5-7% ofTBI patients suffer from at least one seizure [1]. Thepathophysiological mechanisms are not completelyunderstood, and may also differ between early seizures(< 2 weeks of the injury) and the development of post-traumatic epilepsy. This includes the effects of directphysical trauma, excitotoxicity due to iron released fromthe blood [2] and cytokine TGF-b in blood-brain-barrier-mediated activation of astrocytes [3]. In thiswork we propose that a reduction in network connectiv-ity, as presumed present in several cases of TBI, mayresult in seizures and epilepsy.We use a realistic model of neocortex consisting of sixdifferent, multi-compartmental neurons and Hodgkin-Huxley like ion-channel dynamics [4]. Using physiologi-cally realistic connectivityparameters, we analyze net-works of different sizes; ranging from a microcolumn of656 neurons to a mesocolumn that contains 20k neu-rons. In these networks, small lesions are introduced tosimulate axonal and dendritic damage, thereby limitingaction potential propagation. Furthermore, we analyze alumped model of neocortex that is shown to correspondto the detailed model of the microcolumn [5]. Thismodel consists of a system of two differential equationswith two fixed delays. By using an automated parameterestimation method, parameters are identified for whichthe model’s behavior closely resembles that of the realis-tic model. Subsequently, the dependency and sensitivityon these parameters are studied with bifurcation analy-sis. We generate a mesocolumn by linking severallumped units together. Lesions are then introduced bybreaking or reducing some of the connections betweenthe populations. We also study this case using bothanalytical and numerical bifurcation methods.The ratio between excitatory and inhibitory connec-tions is analytically determined as a function of networksize. It is found that, compared to large networks, smallnetworks tend to have a relatively larger number ofexcitatory connections than inhibitory connections. Thissuggests that a lesion splitting the network into smallersub-networks, could increase the ratio of excitatory andinhibitory?connections in a particular sub-network.Choosing parameters that correspond to a region ofmultistability, as determined by the bifurcation analysis,enables us to create an epileptic focus that spreadsepileptiform activity to neighboring areas.ConclusionsBy using multi-scale modeling, large-scale simulations,bifurcation analysis, and parameter estimation, we studythe effects of small lesions in neocortex. From the large-scale simulations we find that“neuronal peninsulas”,created by these lesions, may evolve into epileptogenicnetworks. By studying bifurcations of a lumped modelwith suitable parameters, regions of multistability areidentified that are hypothesized to correspond withepilepsy.