Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Final Activity Report Summary - EHD ATOMIZATION (Electrohydrodynamic atomization methods for the generation of nanoparticles)

This project aims to understand how micro-jets form when strong electrostatic fields are applied to liquid drops (electro-hydrodynamic phenomena (EHD)). Such electrostatically generated micro-jets spontaneously break up into very fine droplets, which have repeatable size and disperse into 'electrosprays'. The conditions that lead to steady (time-independent) droplet formation are of relevance in many applications, specifically in atomization-based methods of nanoparticle fabrication, such as spray-pyrolysis. In this case, EHD techniques have not found much acceptance for various reasons. We are trying to extend the range of liquid properties (chiefly, electrical conductivity) that are compatible with mono-sized droplet generation.

We have thus presented three independent but related studies that aim to clarify, and eventually overcome, these important limitations of EHD spraying. In our first objective, we investigated the 'nanospray' flow regime (nES) as a function of the electrical conductivity of the liquid and other factors. Nanosprays (Wilm-Mann, 1996) are widely known in the practice of mass spectrometry, but have not yet been applied to the manufacture of nanostructured materials. We have found that analogous flow regimes are encountered by nES as by common electrospray methods, and that nES flows are strongly dependent on liquid properties. We also present direct visual proof of the existence of steady cone-jet flows in nES. 'Corona assisted electrospray' flow regime (CAES) has been the focus of our second objective.

Here, EHD flow is believed to be aided by a corona electrical discharge, but this system is considered elusive and difficult to reproduce: it has not been studied after a first report appeared in 1995 (by Tang-Gomez; 'T-G'). Nonetheless, it contains new Physics relating to high surface tension liquids, such as the water. We have focused on reproducing the published data, using pure water electrosprayed into CO2, and on characterizing the combination of control variables (flow rate and capillary tube voltage) allowing stable micro-jets formation.

To our surprise, instead of the two stability domains previously reported (T-G), we have encountered just one. The behaviour of the transmitted electrical current versus the liquid flow rate provides almost conclusive evidence that the flow regime we have encountered is the one identified by T-G as being anomalous, namely CAES, instead of the common electrospray regime, also found by T-G. In the third part of this project, we pursued the understanding of how steady electrified liquid jets break up, namely the deviations from perfect periodicity of breakup of the electrified micro-jets into droplets. This objective was the most challenging of the three, because it focused on the second moments of the droplet size distribution (e.g. standard deviations), rather than first moments (e.g. mean droplet size). Both experimental and computational approaches were pursued.

The experimental work included development of a new measuring technique based on state-of-the-art Differential mobility analyser (DMA) technology. The Computational fluid dynamics (CFD) approach was applied to pneumatically generated steady micro-jets (also called flow-focused jets), which are analogous to electro-hydrodynamic micro-jets. The solutions to the time-dependent Navier-Stokes equations in a 2D-axisymmetric formulation of the problem led to stable micro-jets whose diameters were in good agreement with the theoretical solution and with the experimental values available in the literature.

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