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

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 ‘Arab Spring’ 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 ‘false negative’ of overlooking genuine African social movements and the ‘false positive’ 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