Surface waves on a soft viscoelastic layer produced by an oscillating microbubble

Ultrasound-driven bubbles can cause significant deformation of soft viscoelastic layers, for instance in surface cleaning and biomedical applications. The effect of the viscoelastic properties of a boundary on the bubble-boundary interaction has been explored only qualitatively, and remains poorly understood. We investigate the dynamic deformation of a viscoelastic layer induced by the volumetric oscillations of an ultrasound-driven microbubble. High-speed video microscopy is used to observe the deformation produced by a bubble oscillating at 17-20 kHz in contact with the surface of a hydrogel. The localised oscillating pressure applied by the bubble generates surface elastic (Rayleigh) waves on the gel, characterised by elliptical particle trajectories. The tilt angle of the elliptical trajectories varies with increasing distance from the bubble. Unexpectedly, the direction of rotation of the surface elements on the elliptical trajectories shifts from prograde to retrograde at a distance from the bubble that depends on the viscoelastic properties of the gel. To explain these behaviours, we develop a simple three-dimensional model for the deformation of a viscoelastic solid by a localised oscillating force. By using as input for the model the values of the shear modulus obtained from the propagation velocity of the Rayleigh waves, we find good qualitative agreement with the experimental observations

Correlated and zonal errors of global astrometric missions: a spherical harmonic solution

We propose a computer-efficient and accurate method of estimation of spatially correlated errors in astrometric positions, parallaxes and proper motions obtained by space and ground-based astrometry missions. In our method, the simulated observational equations are set up and solved for the coefficients of scalar and vector spherical harmonics representing the output errors, rather than for individual objects in the output catalog. Both accidental and systematic correlated errors of astrometric parameters can be accurately estimated. The method is demonstrated on the example of the JMAPS mission, but can be used for other projects of space astrometry, such as SIM or JASMINE.Comment: Accepted by AJ, to be published in 201

Human Factors Certification of Advanced Aviation Technologies

Proceedings of the Human Factors Certification of Advanced Aviation Technologies Conference held at the Chateau de Bonas, near Toulouse, France, 19-23 July 1993

Distribution-based bisimulation for labelled Markov processes

In this paper we propose a (sub)distribution-based bisimulation for labelled Markov processes and compare it with earlier definitions of state and event bisimulation, which both only compare states. In contrast to those state-based bisimulations, our distribution bisimulation is weaker, but corresponds more closely to linear properties. We construct a logic and a metric to describe our distribution bisimulation and discuss linearity, continuity and compositional properties.Comment: Accepted by FORMATS 201

Persistent Oscillations of X-ray Speckles: Pt (001) Step Flow

We have performed coherent x-ray scattering experiments on the hexagonally reconstructed Pt (001) surface to study the temperature-dependent surface dynamics. By correlating speckle patterns collected at the (001) anti-Bragg position we are able to measure surface dynamics when the averaged incoherent x-ray scattering appears static. In the temperature range above the rotational epitaxy transition and below the roughening transition (1750 K - 1830 K), we have observed well-defined oscillatory autocorrelations of speckles that persist for tens of minutes, in addition to the expected thermal decorrelation. The observed oscillations indicate surface dynamics due to "step-flow" motion. This is shown with a simple model in which the phase of the scattered x-rays from the steps within the illumination area is retained in the coherent x-ray scattering. This demonstrates a possibility that x-ray speckles can be used to monitor the real-space real-time evolution of surfaces in addition to the traditional decorrelation measurements.Comment: 12 pages, 3 figure