We present an experimental study of ‘polygons’ forming on the free surface of a swirling water flow in a partially filled cylindrical container. In our set-up, we rotate the bottom plate and the cylinder wall with separate motors. We thereby vary rotation rate and shear strength independently and move from a rigidly rotating ‘Newton’s bucket’ flow to one where bottom and cylinder wall are rotating oppositely and the surface is strongly turbulent but flat on average. Between those two extremes, we find polygonal states for which the rotational symmetry is spontaneously broken. We investigate the phase diagram spanned by the two rotational frequencies at a given water filling height and find polygons in a regime, where the two frequencies are sufficiently different and, predominantly, when they have opposite signs. In addition to the extension of the family of polygons found with the stationary cylinder, we find a new family of smaller polygons for larger rotation rates of the cylinder, opposite to that of the bottom plate. Further, we find a ‘monogon’, a figure with one corner, roughly an eccentric circle rotating in the same sense as the cylinder. The case where only the bottom plate is rotating is compared with the results of Jansson et al. (Phys. Rev. Lett., vol. 96, 2006, art. 174502), where the same size of cylinder was used, and although the overall structure of the phase diagram spanned by water height and rotational frequency is the same, many details are different. To test the effect of small experimental defects, such as misalignment of the bottom plate, we investigate whether the rotating polygons are phase locked with the bottom plate, and although we find cases where the frequency ratio of figure and bottom plate is nearly rational, we do not find phase locking. Finally, we show that the system has a surprising multistability and excitability, and we note that this can cause quantitative differences between the phase diagrams obtained in comparable experiments.

We let a steel ball fall on a thin liquid layer. Thereby the liquid was squeezed out from between the falling sphere and the solid boundary, which was made of thick glass, allowing for direct high-speed visualisation of the liquid layer at the point of closest approach. Surprisingly, vapour cavities were created during squeeze, with the liquid forced to change phase to vapour as the sphere approached the boundary and the pressure thus increased. This is a direct contradiction to common preconceptions, where classical theory expects the phase transition from liquid to vapour to occur during depressurisation. We believe that our result is the first direct experimental evidence of the shear-induced cavitation model of Joseph (J. Fluid Mech., 366 (1998) 367).

Microbubbles are used to enhance the contrast in ultrasound imaging. When coated with an optically absorbing material, these bubbles can also provide contrast in photoacoustic imaging. This multimodal aspect is of pronounced interest to the field of medical imaging. The aim of this paper is to provide a theoretical framework to describe the physical phenomena underlying the photoacoustic response. This article presents a model for a spherical gas microbubble suspended in an aqueous environment and coated with an oil layer containing an optically absorbing dye. The model includes heat transfer between the gas core and the surrounding liquids. This framework is suitable for the investigation of both continuous wave and pulsed laser excitation. This work utilizes a combination of finite difference simulations and numerical integration to determine the dependancy on the physical properties, including composition and thickness of the oil layer on the microbubble response. A normalization scheme for a linearized version of the model was derived to facilitate comparison with experimental measurements. The results show that viscosity and thickness of the oil layer determine whether or not microbubble resonance can be excited. This work also examines the use of non-sinusoidal excitation to promote harmonic imaging techniques to further improve the imaging sensitivity.

The sensitivity of ultrasound imaging is greatly enhanced by the use of microbubble contrast agents through resonant volumetric oscillations. While the increased acoustic contrast is of prime interest for perfusion imaging of organs, microbubbles until now have limited benefit in terms of specificity for ultrasound imaging. Original strategies are required to tackle this difficulty that rely on loading functional targeting ligands onto the microbubble encapsulation. In parallel, laser light offers great specificity in its interaction with tissue. This advantage is put to use in photoacoustic imaging where absorbed laser light is converted into a measurable acoustic signal. Here, we present a novel ultrasound contrast agent designed to make use of the specificity of laser light. The acoustic agent consists of a gas core encapsulated by an oil layer containing an absorbing dye. The resulting laser light absorption can then be used to heat up the gas and drive the system into resonance, thereby generating ultrasound. Combining finite difference simulations and ultra high-speed imaging led to a quantitative physical description of the optical and thermal interactions in the system resulting in the efficient generation of acoustic waves in the MHz range. A range of physical bubble parameters are investigated, i.e., thickness and composition of the light absorbing oil layer. This new generation of contrast agents will open up new applications in medical diagnostic and therapeutic imaging.