The morphological path of droplets on a liquid substrate towards equilibrium is investigated experimentally and theoretically. The droplets emerge in the late stage of a dewetting process of short chained polystyrene (PS) dewetting from liquid polymethyl-methacrylate (PMMA). The three-dimensional droplet profiles are obtained experimentally by combining the in situ imaged PS/air interface during equilibration and the ex situ imaged PS/PMMA interface after removal of the PS by a selective solvent. Numerically the transient drop shapes are calculated by solving the thin-film equation in lubrication approximation using the experimentally determined input parameter like viscosity, film thickness and surface tensions. The numerically obtained droplet morphologies and time scales agree very well with the experimental drop shapes. An unexpected observation is that droplets with identical volumes synchronise their motion and become independent of the initial geometry long time before equilibrium is reached.

This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 µm. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample. The displacement of a conductor in a magnetic field induces eddy currents. Conversely, the application of an electrical current in a conductor placed in a magnetic field induces a displacement due to Lorentz force [1]. These two phenomena are currently investigated to produce medical images [2]. In the technique called Lorentz Force Electrical Impedance Tomography [3], also known as Magneto-Acoustical Electrical Tomography [4], an ultrasound beam is focused in a tissue placed in a magnetic field. The displacement of the tissue due to ultrasound in a magnetic field induces an electrical current. The current is measured using electrodes and has been used to produce tissue electrical conductivity interface images. In a "reverse" mode, injecting an electrical current in a tissue placed in a magnetic field induces a displacement due to Lorentz force. As in the megahertz range, shear waves decay over a few micrometers, the displacement propagates only through compression waves. These waves can be detected using ultrasound transducers to produce electrical conductivity images. One implementation of this method is called Magneto-Acoustic Tomography with Magnetic Induction [5].We hypothesized in this study that applying a low frequency (10-1000 Hz) electrical current through a tissue placed in a magnetic field would produce a shear wave within the medium. This could notably have applications in shear wave elastography [6], [7], [8], a medical imaging technique used to map the mechanical properties of biological tissues. The mechanical properties of biological tissues are known to be viscoelastic (hence frequency-dependent) [9], [10], [11], often anisotropic, e.g. along muscle fibers [12], and nonlinear (changing with pre-stress). However, in the field of medical imaging, most applications rely on a simple model, assuming an elastic isotropic linear solid. The viscoelasticity effect has been shown to have only second effect orders [13] and the synthetic phantoms as used in this study can reasonably be considered as fully isotropic and linear [14], [15]. Under these assumptions, tissue elasticity can be described by two parameters only, e.g. the shear modulus µ and Poisson's ratio. The shear modulus ...

The Lorentz force can be used by different means to image thermal lesions in biological tissue. In the first method presented here, so-called magneto-acoustical electrical tomography, a tissue sample is held in a magnetic field and is subsequently exposed to a focused ultrasound beam. The displacement within the magnetic field caused by this ultrasound beam results in an electrical current due to the Lorentz force. In this way, the change in electrical conductivity due to the presence of thermal lesions can then be observed. Conversely, when an electrical current is applied to tissue placed in a magnetic field, a shear wave is induced by the Lorentz force. Elastography images can be reconstructed from this shear wave, revealing thermal lesions by the change in elastic modulus. The first method was tested on ex-vivo chicken breast sample with a 500 kHz transducer and a 300 mT magnetic field. The second method was tested on gelatin phantom with a 100 mA current and 300 mT magnetic field. Images and results will be presented for both methods. These techniques could be used for the monitoring of thermal lesion formation in high intensity focused ultrasound treatment.