Abstracts - KTH Mechanics

Generation of metal nanodroplets and impact experiments
A. Habenicht ∗ ,P.Leiderer ∗ andJ.Boneberg ∗
Flat metal nanostructures on inert substrates like glass, silicon or graphite are
illuminated by single intensive laser pulses with fluences above the melting threshold.
The liquid structures produced in this way are far from their equilibrium shape and
a dewetting process sets in. On a timescale of a few nanesconds, the liquid contracts
toward a sphere. During this contraction the center of mass moves upward, which can
lead to detachment of droplets from the surface due to inertia. The velocity of the
detaching nanodroplets is measured with a light barrier technique 1 .Theexperiment
shows (see figure 1a) that the velocity of the detached droplet is constant over a
large range of laser energy densities. This supports the model of a dewetting driven
process: The droplet gains surface energy by contracting toward a sphere which is
then converted into kinetic energy. Results are shown for different materials, where
different surface tensions result in different velocities.
Further we show impact experiments where the droplets are landed on another
substrate. The particles cool down during the flight due to thermal radiation. They
solidify either during flight or when impacting on the substrate. By catching the particles
at different distances, the landing temperature can be varied. A droplet which
is still molten at impact, spreads (and, under certain circumstances, even rebounds)
until the heat loss into the substrates leads to solidification of the particle. Snapshots
of different stages of droplet impact are shown (see figure 1b).
∗ University of Konstanz, Dept. of Physics, LS Prof. Dr. Leiderer, D-78457 Konstanz, Germany.
1 Habenicht et al., Science 309, 2043 (2005).
Velocity (m/s)
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a) b)
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Laser energy density F/FC Figure 1: (a) Velocity of the detached gold particles as a function of the energy density
of the melting laser. The velocity is constant of a large area of energy densities due
to conversion of surface energy into kinetic energy (b) Impact of gold droplets with
different sizes on silicon. Depending on the landing temperature (and thus the size of
the particle) the droplets solidify at different stages (shown on the right) of droplet
impact.
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