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Transient dynamics of a Leidenfrost vapor layer in drop impact is studied using x-ray imaging. It is shown that a vapor disk first forms, growing in thickness, following Fourier's law. At a certain thickness, ripples generate near the edge and propagate to the center by capillary waves, improving evaporation.

The evolution of Fe surface morphology during heteroepitaxial growth on Si(111) and Si(001) substrates was investigated using real-time synchrotron x-ray reflectivity measurements. The growth on the Si(111) surface was divided into the initial stage heteroepitaxial regime, the intermediate stage crossover regime, and the final-stage homoepitaxial regime. The evolution of the surface roughness in the late stage growth was described by the dynamic scaling exponent of $\ensuremath{\beta}\ensuremath{\sim}0.24$ consistent with reported values. On the Si(001) surface, an interlayer was formed prior to the growth of a nonepitaxial Fe layer. The roughness evolution of the Fe/Si(001) was described by $\ensuremath{\beta}\ensuremath{\sim}0.36.$

During bubble or droplet coalescence, there is a puzzling tendency for the coalesced bubble or droplet to be preferentially placed closer to the larger of its two parents. We confirm that this preference is a function of parent size ratio by directly visualizing coalescing air bubbles on an oil-water interface and coalescing water droplets immersed in oil. We find that the final position of the coalesced sphere is controlled by surface energy release and is related to the parent size ratio by a power-law relationship.

We show direct evidence that focused x rays enable us to merge polymer colloidal particles at room temperature. This phenomenon is ascribed to the photochemical scission of colloids with x rays, reducing the molecular weight, glass transition temperature, surface tension, and viscosity of colloids. The observation of the neck bridge growth with time shows that the x-ray-induced colloid coalescence is analogous to viscoelastic coalescence. This finding suggests a feasible protocol of photonic nanofabrication by sintering or welding of polymers, without thermal damage, using x-ray photonics.

When a coffee drop dries on a solid surface, it leaves a ringlike deposit along the edge and this is known as the "coffee-ring effect." We find a different motion of particles repelling the coffee-ring effect in drying droplets; the motion of particles that is initially toward the edge by the coffee-ring effect is reversed toward the center by a capillary force. The reversal takes place when the capillary force prevails over the outward coffee-ring flow. We discuss the geometric constraints for the capillary force and the reverse motion. Our findings of reversal phenomena would be important in many scenarios of drying colloidal fluids.

We discovered that intense irradiation by hard-x-ray strongly decreases the effects of natural surface tension of water in droplets and capillary tubes. The effect was revealed by direct experimental observations with phase contrast microradiology. A model based on ionization and surface charging explains this so far undetected phenomenon. The effect can impact the results of many experimental techniques based on x rays. This is an example of the largely unexplored effects that can be produced by extreme intense x-ray irradiation-an important issue due to current development of x-ray free-electron-lasers with unprecedented brilliance.

Keywords: CIBM-PC Reference EPFL-ARTICLE-186229doi:10.1103/PhysRevB.76.064117 Record created on 2013-04-25, modified on 2017-05-12