Electrospraying route to nanotechnology: An overview

Abstract

Electrospraying (electrohydrodynamic spraying) is a method of liquid atomization by means of electrical forces. In electrospraying, the liquid at the outlet of a nozzle is subjected to an electrical shear stress by maintaining the nozzle at high electric potential. The advantage of electrospraying is that droplets can be extremely small, in special cases down to nanometers, and the charge and size of the droplets can be controlled to some extent by electrical means, i.e., by adjusting the flow rate and voltage applied to the nozzle. Due to its properties, electrospraying is considered as an effective route to nanotechnology. The paper considers the latest achievements in micro- and nano-thin-film production, including self-assembled nanostructures, in solid nano-particle generation, and in the formation of micro- and nanocapsules.

Introduction

Electrospraying (electrohydrodynamic spraying) is a method of liquid atomization by means of electrical forces. In electrospraying, the liquid flowing out of a capillary nozzle, which is maintained at high electric potential, is forced by the electric field to be dispersed into fine droplets. Electrospray systems have several advantages over mechanical atomisers. The size of electrospray droplets can range from hundreds micrometers down to several tens of nanometer. The size distribution of the droplets can be nearly monodisperse. Droplet generation and droplet size can be controlled to some extent via the flow rate of the liquid and the voltage at the capillary nozzle. The fact that the droplets are electrically charged facilitates control of their motion (including their deflection and focusing) by means of an electric field. Charged droplets are self-dispersing in space, resulting also in the absence of droplet coagulation. The deposition efficiency of a charged spray on an object is higher than for an un-charged spray. This feature can be advantageous, for example, in surface coating, thin-film production, or electroscrubbing.

Electrospraying can be widely applied to both industrial processes and scientific instrumentations. The interest in industrial or laboratory applications has recently prompted the search for new, more effective techniques which allow control of the processes in which the droplets are involved. Electrospraying has opened new routes to nanotechnology. Electrospray is used for micro- and nano-thin-film deposition, micro- or nano-particle production, and micro- or nano-capsule formation. Thin films and fine powders are (or potentially could be) used in modern material technologies, microelectronics, and medical technology. Research in electro-microencapsulation and electro-emulsification is aimed at developing new drug delivery systems, medicine production, and ingredients dosage in the cosmetic and food industries. Electrohydrodynamic spinning (electrospinning) of viscous liquids facilitates the production of nanofibers for masks, filters, scaffolds for biological tissue, and intelligent garment manufacturing. Recent advancements in nanoelectrospray technologies have been briefly reviewed by Salata [1].

This paper reviews many aspects of these emerging applications of electrospray in nanotechnology. In this paper, the term nanotechnology refers to the processing, production, or application of materials and structures having size scale less than 100 nm. Thus nanotechnology includes the investigation of physical phenomena at supra-atomic levels, the creation of atomic clusters/compounds (e.g. polymers, quantum dots, molecular switches, carbon nanotubes, fullerenes of the size <10 nm), generation of nanoparticles (nanograins, nanocapsules of diameter <100 nm), deposition of nano-thin films (nanocoatings, self-assembling monolayers, nanowires of thickness <100 nm), spinning of nanofibers (carbon fibers, polymer fibers, textiles, nanoyarns of diameter <1000 nm), and production of nanostructured materials (nanocomposites which are conjunction all of above). There are two main approaches to the nanotechnology: “top-down” and “bottom-up”. The “top-down” approach refers to physical or chemical machining of a bulk material down to nanometer scale by grinding, milling, etching, or lithographing. The “bottom-up” approach involves building a nanostructure from elementary components via molecule-by-molecule or grain-by-grain deposition onto a substrate, epitaxial growth, plating, and intercalation or implantation. Among these techniques, electrospraying is placed among the “bottom-up” techniques because the building blocks are produced from fine droplets after solvent evaporation.