Key Research Interests and Expertise

High-Resolution Electrostatic Inkjet Printing

While inkjet printing has proven suitable for many direct writing applications, the size of the generated features cannot easily be reduced below a few tens of micrometers. It is possible to obtain smaller feature sizes by scaling the size of the inkjet nozzle down to micrometer diameters but this leads to nozzle clogging. The accuracy of droplet placement is limited by the free trajectory of the ejected droplet which is vulnerable to disturbances. As a result, the best printing accuracy so far reported has been 5mm. Inkjet printing as a direct writing method for electronics is therefore limited by the problems of multi-layer accuracy and the difficulty of avoiding clogging if smaller drops are needed. Clearly then there exists a capability gap between the high resolution at high costs of photolithography and the low cost but low resolution of inkjet printing.

Electrospray occurs when the electrostatic force on the surface of a liquid overcomes the surface tension and is most commonly used today as a source of gas phase ions in mass spectrometry, where it has transformed the analysis of large biomolecules. The most studied and most stable form of the process is the cone-jet regime, wherein the balance between electrostatic stresses and surface tension creates a Taylor cone, from the apex of which a liquid jet is emitted. The use of standard electrospray as a direct printing technique would have to overcome severe technical obstacles. The foremost of these is flow control as in traditional electrospray operation the supply of fluid to the spray cone is controlled either by a pump or by using a pressure head and cannot be rapidly controlled to stop or start the spray.

Nanoelectrospray, in its simplest form, is when the electrospray flow rate is exclusively controlled by the applied voltage. One of the benefits of nanoelectrospray is that very low flowrates are easily achieved by selection of the correct nozzle and liquid combination. In previous work we identified new modes of electrospray called pulsating nanoelectrospray (Alexander MS et al, 2006) which allows the controlled ejection of minute volumes of liquid, as low as sub-femtoliter, which may be used to pattern surfaces with micron-sized features (Paine,  M. Alexander, MS. Stark, JPW. 2007)

This equates to a 10-fold advancement in print resolution over current inkjet technology and our research focus is on the further development of the nanoelectrospray printing technique as a key enabling tool for numerous applications

Figure 1 A) High speed images of jet formation and collapse on a 50um nozzle, B) Fluorescent microscopy image of patterned 2D albumin feature with line width of ~8µm


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