Droplet Coalescence

Two recent PRLs are addressing the issue of how two droplets merge into one.

Kamel Fezzaa and Yujie Wang from Argonne use ultrafast x-ray phase-contrast imaging to take sub-microsecond exposures of droplet coalescence, which is complete in just under a milisecond.

Phys. Rev. Lett. 100, 104501 (2008)

The studied liquid droplets are ~1mm (bar size in the image on the left) in size, and can be seen with <5 micron resolution using phase contrast (as opposed to adsorption contrast) using high energy 13keV x-rays. Fezzaa and Wang cleverly used the hybrid filling pattern of Advanced Photon Source, where each electron bunch produces a short x-ray pulse 472 nanoseconds long used for imaging, with each pulse separated from the next one by 3.6 microseconds. The result is a sub-microsecond “shutter time” defined by the length of each pulse, with consecutive images taken 3.6 microseconds apart.

Of particular interest in this study is the stability of torroidal air bubble formed due to air trapped by the two rapidly coalescing droplet menisci. Fezzaa and Wang show for the first time that the torroidal bubble remains trapped until some 400 microsecond after the droplets start merging.

The second recent paper on this topic of droplet coalescence is by Sara Case and Sid Nagel at University of Chicago. PRL 100, 084503 (2008)

Case and Nagel abandon the visual approach to studies of ultrafast coalescence process, and instead adopt a technique which measures the changes in conductivity across the connection between the two droplets as a function of time. When the droplets begin to coalesce, the effective resistivity is high, since it is defined primarily by the width of the narrow region where the two droplets touch each other. As they coalesce, this resistivity will drop. This technique proves to be especially useful in the timescale range from sub-microsecond to hundreds of microsecond. Case and Nagel observe a cross-over in power-law behavior for R(t) from 1/t for small t to 1/√t at large t, but do not see time-dependent fluctuations in R(t) which would be the signature of the connected menisci repeatedly disconnecting and reconnecting again.

Metal-Insulator transition in VO2

There were a number of papers in the past month or so on Metal-Insulator transition in vanadium dioxide. The first work is the near-scanning infrared microscopy measurements done by Mumtaz Qazilbash et al. from Basov group at UCSD published last week in Science revealing nucleation and growth of metallic domains in vanadium dioxide films with nanoscale resolution.

M. M. Qazilbash et al., “Mott Transition in VO₂ Revealed by Infrared Spectroscopy and Nano-Imaging” Science 14 December 2007: 1750-1753

Last month there was another Science paper by Peter Baum et al. from Zewail group at Caltech looking at “4D” visualization of metal-insulator transition in vanadium dioxide – involving a combination of 3D observations of Bragg peaks with electron diffraction, resolved on the femtosecond scale.

Also see this PNAS article by Baum and Zewail (PNAS | November 20, 2007 | vol. 104 | no. 47 | 18409-18414, Open access), as well as a Science perspective by Cavalleri.

Baum et al., Science 318 (5851), 788. [DOI: 10.1126/science.1147724]

And in September, there was a PRL by Kubler et al. on light-induced femtosecond MIT transition in VO2, followed by another PRL by Hilton et al. in late November on a similar topic.