Droplet Behaviour in Inkjet Printing
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Droplet Behaviour in Inkjet Printing Jonathan E Stringer, Patrick J Smith and Brian Derby School of Materials, University of Manchester, Manchester, M1 7HS United Kingdom. ABSTRACT The interaction of an inkjet printed droplet with a substrate is of importance when determining the final size and shape of deposits. A simple model is proposed that relates droplet diameter, printed dot pitch and equilibrium contact angle to as-printed track width. Reasonable agreement was found between the model and final track width, with slight over-prediction accounted for by removal of the organic component of the ink during heat treatment. Immediately after droplet impact, the drop may spread to a considerably greater extent than predicted by equilibrium because of the kinetic energy of the droplet in flight. Simulations of droplet impact showed that the maximum droplet spread decreased linearly with equilibrium contact angle. Recoil of these droplets towards their equilibrium shape occurred above a threshold contact angle. This threshold was greater than expected, suggesting an energy barrier preventing recoil or the kinetics of recoil being too slow for recoil to occur within the timeframe of this study. INTRODUCTION Inkjet printing can be used as a solid freeform fabrication technique whereby droplets are placed at locations determined by a computer aided design (CAD) file. These droplets coalesce to form objects. This proceeds by a train of droplets merging to form a line, adjacent lines join to form a 2-dimensional pattern and finally sequential layers fuse together to form the desired object. Thus, the size and shape of the deposited droplet determines the resolution at which an object is fabricated. Droplet deformation upon impact with a substrate is driven by its change in kinetic and total surface energy, with the resulting deformation impeded by viscous dissipation. Most droplet impact studies in the literature have been carried out with droplets that are either over an order of magnitude larger than those of inkjet printing [1] or possess a much greater velocity [2]. Under these impact conditions the kinetic energy is normally much greater than the surface potential energy of the undeformed shape and hence this dominates the resulting deformation. However, under the impact conditions used in inkjet printing (droplet velocity 1 - 10 ms-1 and droplet mass approximately 100 ng) the kinetic and total surface energy are of similar magnitude at impact. The initial total surface energy of a spherical droplet is given by πd2σLV, where d is the droplet diameter and σLV is the specific liquid/vapour surface energy of the liquid. At equilibrium a droplet in contact with a substrate will form a spherical cap of contact angle, θ, and base diameter, deqm. The equilibrium contact angle is determined by the balance of the surface energies of the free droplet surface, the free substrate surface and the droplet/substrate interface, as expressed by the Young-Dupré equation. Assuming conservation of mass, the ratio, βeqm, of the sp
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