Theory predicts that dewetting of a homogeneous liquid film from a solid surface may proceed via unstable surface waves on the liquid. This phenomenon, usually termed spinodal dewetting, has been sought after in many systems. Observations in liquid crystal and liquid metal films showed that, as expected, the emerging structures were similar to those found for spinodal decomposition in mixtures. Certain differences, however, could be attributed to peculiarities of the wetting forces in these two dissimilar systems, thereby demonstrating the role of nonlinearities inherent in the wetting forces.

We have studied the dewetting of thin liquid metal films (Au, Cu, Ni) on fused silica substrates which occurs after melting with a Q-switched laser pulse. Optical microscopy, scanning electron microscopy, and scanning near field acoustic microscopy reveal two distinctly different modes of the dewetting process: On one hand, we observe heterogeneous nucleation of "dry" circular patches, which grow in diameter during the melting period. On the other hand, an instability of the liquid film against the growth in amplitude of surface waves with a characteristic wavelength is observed, which we believe is the first observation of spinodal dewetting. In contrast, the final structure of the ruptured film depends on whether nucleation or spinodal dewetting is dominant. [S0031-9007(96) PACS numbers: 68.45. Gd, 47.20.Ma, 61.25.Mv, 68.15.+e Dewetting of metastable thin films from a solid substrate is currently a topic of great interest [1][2][3][4]. This is based not only on the applicational relevance, e.g., in thin film technology. Also from a fundamental point of view there are unsolved questions concerning the interpretation of experimentally observed phenomena with respect to the behavior predicted theoretically. Particularly interesting is the self-organized structure evolution in time and space. Often the basics of dewetting have been studied on liquid films because heterogeneous influences, i.e., from grain boundaries or stresses, associated with solid films are not as prominent or must not be considered. From the theoretical considerations, dewetting of a metastable liquid film can develop via two different mechanisms [5]: First, nucleation and growth of holes can take place. This process has already been studied in detail experimentally. Redon et al. [4] nucleated holes in thin films of alkanes on silicon wafers at temperatures above the glass transition and studied their growth as a function of the surface tension of the alkanes. In general the growth of the holes leads to accumulation of the material along the perimeter of the holes by building up an elevated rim around them. The second process which can lead to dewetting is based on an instability of the film against thermally activated surface waves. According to theory [5][6][7], this instability ruptures the film spontaneously and a characteristic wavelength of surface modulations with the minimal risetime should dominate for a given system. This characteristic wavelength should scale with the square of the liquid film thickness [5]. However, the experiments reported so far do not provide an unambiguous demonstration for this behavior. Polystyrene films dewetting from silicon surfaces were found to develop circular holes whose number density scaled with the film thickness [8]. This was taken as evidence for spinodal dewetting as the process responsible for their formation. However, no direct indication for unstable surface waves was found, which in a spinodal process should precede the breakup of the holes. Guerra et al. [9] observed rather ordered wave...

Transient acoustic waves generated by laser-induced bubble formation at a liquid-solid interface are sensitively monitored using optically excited surface plasmons. This method enables the detection of both the compressive and tensile waves with high accuracy as demonstrated for the propagation and reflection of acoustic pulses at a quartz-water interface. Unique advantages of this new technique are the high sensitivity of 0.1-0.2 MPa that could be achieved for absolute pressure measurements on a nanosecond time scale and its ability to probe exact pulse profiles due to the localized probe depth of surface plasmons.

Abstract. Bubble nucleation and growth dynamics, on a nanosecond time scale, induced by pulsed laser heating of a liquid-solid interface are studied experimentally. A surface-plasmon probe is implemented as a novel, highly sensitive method for the study of submicroscopic bubbles, providing accurate information on the nucleation thresholds, growth velocities, and transient pressure generation by rapid bubble growth. Owing to the higher sensitivity of the surface plasmon probe to small bubbles, it is demonstrated that bubble nucleation sets in at a lower liquid superheating than previously determined with the use of optical reflectance or piezoelectric transducer measurements. A comparison of experimentally determined bubble growth velocities with computational results confirms that bubble growth is governed by the heat transfer from the solid surface into the liquid. Reconstructed surface plasmon resonance curves from transient signals are used to estimate the fractional volume and number density of bubbles in the superheated liquid layer. Further, a surface plasmon probe is utilized for the absolute measurement of bubble-growthinduced pressure amplitudes on a nanosecond time scale. The measurements yield peak pressure amplitudes in the range of ∼ 1-5 MPa with a pressure pulse width of ∼ 40 ns. Additionally, the phase of an acoustic pulse is observed to change upon reflection at the liquid-solid interface if bubbles are present, providing a direct proof for laser-induced bubbles.