5 research outputs found
Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces
Droplets impacting on a superheated surface can either exhibit a contact
boiling regime, in which they make direct contact with the surface and boil
violently, or a film boiling regime, in which they remain separated from the
surface by their own vapor. The transition from the contact to the film boiling
regime depends not only on the temperature of the surface and kinetic energy of
the droplet, but also on the size of the structures fabricated on the surface.
Here we experimentally show that surfaces covered with carbon-nanofibers delay
the transition to film boiling to much higher temperature compared to smooth
surfaces. We present physical arguments showing that, because of the small
scale of the carbon fibers, they are cooled by the vapor flow just before the
liquid impact, thus permitting contact boiling up to much higher temperatures
than on smooth surfaces. We also show that, as long as the impact is in the
film boiling regime, the spreading factor of impacting droplets follows the
same \We^{3/10} scaling (with \We the Weber number) found for smooth
surfaces, which is caused by the vapor flow underneath the droplet.Comment: 10 pages, 6 figure
Erratum:How water droplets evaporate on a superhydrophobic substrate (Physical Review E - Statistical, Nonlinear, and Soft Matter Physics (2011) 83 (026306))
Hybrid social movements in Africa
This essay identifies patterns in the organisation and character of social movements in Africa, drawing upon examples from sub-Saharan Africa and finding connections with the 2011 āArab Springā uprisings. It pays attention to historicising social movements, global linkages and the problem of sustaining change. Rather than defining social movements in an a priori manner, or generalising from definitions derived from the western societies, it explores their concrete meanings in Africa. Thus it aims to avoid both the āfalse negativeā of overlooking genuine African social movements and the āfalse positiveā of labelling movements in a misleading manner. It identifies constraints upon collective action in Africa, exploring the political dynamics which undermine the formation of durable and organised movements and limit their capacity to represent popular concerns