December 2, 2008 (Tuesday)
11:00 am in Howey N110

"Whipping instability in electrified micro/nanojets"

Antonio Barrero, University of Sevilla (Spain)

Micro/nanoparticles of interest in many technological applications can be efficiently generated from capillary liquid flows. Most of these techniques are based in the deformation and stretching of a liquid thread up to microscopic or even nanometric dimensions by forces of different nature (mechanical, electrical). Among those techniques employing electrical forces, the electrospray is the most popular since is able to produce particles in the nanometric range. The electrospray technique is based in the generation of charged micro or nanometric jets which eventually break up into droplets, due to capillary instabilities, giving rise to a charged aerosol. Under some circumstances, the charged capillary liquid jet may develop a special type of non-symmetric instability, usually referred in the literature as whipping mode. This instability plays a key role in the generation process of nanofibers by electrospinning (P. K. Baumgarten, J. Colloid and Interface Sci., 36, 1971). Whipping instability was first investigated some decades ago by Taylor (G. I. Taylor, Proc. Roy. Soc. London, 1969) and more recently by others (Hohman et al. Phys. of Fluids, 13, 2001; Reneker, Yarin et al., J. of Appl. Phys., 87, 2000). The developed models are able to give a qualitative understanding of the phenomenon but none of them seems to be sufficiently accurate when they are compared with experimental results. Whipping instability of charged jets usually manifests itself as a series of fast and violent lash, which makes difficult the characterization of the instability in the laboratory. Whipping instability also develops when electrosprays are operated in a dielectric liquid bath, instead than in air (Barrero et al., J. Colloid & Interf. Sci., 272, 104-108, 2004). Although the phenomenon is essentially the same in both media, in the case of a dielectric liquid the required electric field is smaller than in air; the violence of the lashes being also much smaller. Taking advantage of this situation, we have carried out a detailed experimental characterization of the instability of a charged jet of glycerol dispersing in a bath of hexane. In particular, making use of high speed cameras, we have measured some parameters characterizing the whipping as a function of the two governing parameters: the applied electrical voltage and the flow rate of the jet.