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We study the motion of a drop lying on a plate simultaneously submitted to horizontal and vertical harmonic vibrations. The two driving vibrations are adjusted to the same frequency, and, according to their relative amplitude and phase difference DeltaPhi, the drop experiences a controlled directed motion with a tunable velocity. We present a simple model putting in evidence the underlying mechanism leading to this ratchetlike motion of the drop. Our model includes the particular case DeltaPhi=pi corresponding to the climbing of a drop on a vertically vibrated inclined substrate, as recently observed by Brunet et al. [Phys. Rev. Lett. 99, 144501 (2007)10.1103/PhysRevLett.99.144501]. This study gives insights in the fundamental issue of wetting dynamics and offers new possibilities of controlled motion in droplet microfluidics applications.

Nonthermal generation of coherent-acoustic phonons is observed in metallic nanoparticles of tin and gallium, which are solid and liquid, respectively, at room temperature, by applying femtosecond pump-and-probe spectroscopy. Oscillations in the photo-induced differential reflection are clearly detected in the picosecond time domain. The measured period increases approximately linearly with size over a wide range and extracted sound velocities are consistent with those of bulk. Transition to ballistic regime in carrier excitation is also discussed on the basis of the disappearance of oscillations for particle sizes below hot-carrier mean free paths.

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