13:30
"Science in the CDLCME"
Dr. Robert Mettin
Drittes Physikalisches Institut, University of Göttingen, Germany
The scientific mission of the research project „Christian Doppler Laboratory for Cavitation
and Micro-Erosion“ is presented, and the three Modules on “Acoustic streaming and acoustic
cavitation”, “Single-bubble-induced flow” and “Liquid-jet-induced flow” are briefly
reviewed.
13:45
“Measurement and quantifying parameters for cavitation”
Dr. Christian Koch
Physikalisch Technische Bundesanstalt, Braunschweig, Germany
Cavitation is used in many technical and medical applications.
Because of the stochastic behaviour, devices and technical processes
are designed empirically and output control and optimization are
difficult. The measurement of global parameters that represent
cavitation effects used in the application allows a quantitative
description of the processes involved. The talk presents results of
sound pressure measurements in cleaning baths and the derivation of
useful parameters from the measurement. It is shown that sound field
parameters are correlated to cavitation effects. For the
determination of these effects useful measurement strategies are
demonstrated and it is shown, that a quantitative description of
cavitation fields used in practise is possible.
14:15
“Non-stationary boundary layers or why bubbles attract dirt”
Prof. Claus-Dieter Ohl
Nanyang Technological University, Singapore
In ultrasound cleaning oscillating gas bubbles are responsible for the
efficient removal of dirt from surfaces. However, very little is known on
the fluid mechanics involved. We approach this problem by simplifying the
complex, fast, and microscopic interaction and study the dynamics of a
particle next to a single oscillating bubble. This single bubble is created
with a laser pulse and the bubble-particle interaction is recorded with
high-speed photography. The particle trajectory is measured as a function of
the initial separation and the particle diameter. For all particles
investigated we find the same intriguing dynamics: only at very short
distances a fast acceleration away from the bubble is observed while at
larger separations the particle is attracted towards the bubble. Thus most
of the particulate contaminants are attracted towards the bubble and they
will approach a stable particle-bubble separation. Interestingly, once
non-dimensionalized the particle trajectories collapse onto a single master
curve. Using a force balance approach including a model for the boundary
layers we find good agreement with the experiments. The developing unsteady
boundary layers are causing the attraction of the particles. Additionally,
we observed rolling dynamics induced by the strong shear flow and find
angular speeds of more than 100,000 revolutions per second! In summary both
-torque and linear drag- are contributing to the acceleration of adherent
dirt particles and are thus important for cleaning with bubbles.
15:00
“Laser-induced micro- and nano-bubbles: Basic physics and biomedical applications”
Prof. Alfred Vogel
Institut für Biomedizinische Optik, Universität zu Lübeck, Germany
Optical breakdown and spherical cavitation bubble collapse are different nonlinear
mechanisms producing a large energy density within a small spatial region. Extreme states of
matter with pressures up to 107 MPa have been claimed for femtosecond optical breakdown
[1], and temperatures up to 108 K for the collapse of bubbles in a sound field were postulated
shortly after the discovery of sonolominescence [1,2]. Direct experimental verification of such
claims is impossible because of the small spatial and temporal scale on which the extremes
occur. Instead, p and T have to be inferred from other experimental information and equation
of state (EOS) data.
We managed to produce highly spherical cavitation bubbles in water by focusing laser
pulses at large numerical aperture through long-distance water immersion objectives [3]. This
enables to directly compare states of matter produced by optical breakdown and bubble
collapse. Photographs of the plasma luminescence provide the plasma size, transmission
measurements the absorbed energy, and both data together the plasma energy density. The
bubble dynamics is measured using a continuous probe beam in transmission or reflection
mode. Because of their perfectly spherical shape, small bubbles of up to 40 µm radius do not
decay upon collapse but we observe up to 100 after-bounces. The maximum bubble radius
during the first oscillation cycles is derived from the respective oscillation times, and the
equilibrium radius Rn of the gas bubble into which the oscillating cavitation bubble evolves is
deduced from the oscillation frequency at later times.
We found that energy deposition in femtosecond breakdown as well as nanosecond
breakdown at UV and visible wavelength can be precisely controlled such that the resulting
bubble size can be tuned from tens of nanometers to millimeters. This tunability is a
prerequisite for many applications ranging from transient cell membrane permeabilisation for
gene transfer to precise tissue dissection in corneal refractive surgery. I shall discuss these
applications, and present a tunability map for controlled nonlinear energy deposition in the
(wavelength/pulse duration) parameter space.
For Femtosecond breakdown well above the bubble formation threshold, we investigated
energy density, temperature, and pressure upon bubble generation and spherical collapse.
Femtosecond optical breakdown is isochoric because energy deposition occurs faster than
thermal expansion. Therefore, the liquid density 0 at the end of the pulse is known, and from
0 together with the measured value for the plasma energy density, we can calculate pressure
and temperature using EOS data for high energy densities [4]. For  = 23 kJ/cm3 at 160 nJ
pulse energy (7 above threshold), we obtain T = 2800 K and p = 4.8103 MPa. Using the
Gilmore model, we calculated the bubble dynamics for the experimentally determined values
of plasma radius R0 = 1.3 µm, maximum bubble radius Rmax = 36 µm, and equilibrium radius
Rn = 2.45 µm. The predicted breakdown pressure is 1.26103 MPa, and the collapse pressure
4.1104 MPa. The pressure upon spherical bubble collapse is ≈ 10 times larger than the
breakdown pressure because the collapsed bubble (Rmin = 90 nm) is much smaller
than the breakdown region (R0 = 1.3 µm).
We conclude that states of matter upon femtosecond breakdown are much less extreme
than postulated in [1]. However, the collapse of spherical fs laser-produced vapor bubbles
produces larger pressures than fs optical breakdown, and likely also larger pressures than
spherical
gas
bubble
collapse
in
a
sound
field
(SBSL)
[2].
By contrast, for nanosecond optical breakdown well above threshold we found experimental
evidence for extreme energy densities  270 kJ/cm3, leading to X-ray emission in the water
window (2.4 – 5 nm).
References
1 S. Juodkazis et al.:”Laser-induced micoexplosion confined in the bulk of a sapphire crystal: evidence of multimagabar pressure,” Phys.
Rev. Lett. 96, 166101 (2006).
2 M. Brenner, S. Hilgenfeldt, D. Lohse, ”Single bubble sonoluminescence,” Rev. Mod. Phys. 74, 425-484 (2002).
[3] A. Vogel, N. Linz, S. Freidank, and G. Paltauf, "Femtosecond laser-induced nanocavitation in water: implications for optical breakdown
threshold and cell surgery," Phys. Rev. Lett. 100, 038102 (2008).
4 T. R. Mattsson, M. P. Desjarlais, “Phase diagram and electrical conductivity of high energy-density water from density functional
theory,” Phys. Rev. Lett. 97, 017801 (2007).
15:30
Droplet impact: compressibility and wetting effects
Prof. Martin Rein
Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR), Göttingen,
Germany
High-speed impacts of droplets are well-known to cause serious erosion to surfaces. One reason are
high pressures occurring on impact. The importance of shock waves for the generation of extreme
pressure peaks will be explained based on a classical shock polars approach for the case of liquidliquid impacts. After impact, expansion waves propagating within the droplet may result in the
formation of cavitation bubbles whose collapse can also contribute to erosion.
At low velocities droplets impinging on a dry solid surface can spread on the surface or splash. It is
shown that critical conditions of splashing inception, the so-called splashing threshold, have often
been obtained under slightly different experimental conditions and are partly contradictory.
Furthermore, no general agreement on the basic physical mechanism causing splashing does exist.
After considering those parameters that are likely to govern splashing in a dimensional analysis,
limiting conditions of splashing are re-evaluated. Information drawn from these considerations is used
to distinguish between different splashing thresholds. For smooth surfaces there is evidence that the
wetting behaviour of the liquid plays an important role for the inception of splashing. A mechanism
causing the onset of splashing on smooth surfaces is proposed that may also explain the well-known
fingering of the liquid lamella formed after impact.
16:15
“What are the Fundamental Limits and Practical Applications of
Sonoluminescence”
Prof. Seth Putterman, University of California, Los Angeles (USA)
Sonoluminescence is an extraordinary nonlinear oscillation in which a bubble concentrates
acoustic energy density by 12 orders of magnitude to make a flash of ultraviolet light which
can be as short as 35.picoseconds. Time resolved spectra reveal the formation of a Planck
blackbody spectrum even when the photon- electron mean free path is much bigger than the
micron plasma which forms inside the imploded bubble. Sonoluminescence is the means for
synchronizing detector arrays in neutrino observatories. It is used extensively for aesthetic
surgery, and at 1MHz its use for surgery at a distance is proposed. Sonoluminescence is an
ideal test-bed for high energy density simulations. Can cavitation be used to build the first
desk-top thermonuclear neutron generator? So far we are about a factor of 10-100 in
temperature below the point of success However, the phonon spectra of ferroelectrics, and
triboluminescence – the mother of sonoluminescence- both facilitate the construction of
highly compact neutron and x-ray sources.