3D Printed Microdroplet Curing: Unravelling the Physics of On-Spot Photopolymerization

Droplet-based direct-write printing (one form of 3D printing) methods, like inkjet printing, aerosol jet printing (AJP), etc., have changed our ideas about bottom-up additive manufacturing. AJP is capable of 3D printing by depositing microdroplets that lead to certain benefits in 3D printing of microscale structures. Here, we study the in situ photopolymerization-based curing of a microdroplet. The droplet is simultaneously spreading and curing, which leads to the rise of two time scales, the spreading time scale (τ_s) and the photopolymerization time scale (τ_p). When τ_s ≪ τ_p, the spreading occurs very fast and is independent of polymerization. If the time scales are of the same order (τ_s ∼ τ_p), the spreading is significantly affected by the polymerization. For this case, a progressive increase in the viscosity of the drop during the spreading ensures a much slower spreading and also a much weaker extent of spreading (i.e., the spreading ceases at a much smaller spreading radius of the drop). This complex thermofluidics is eventually manifested as distinct differences in the time-dependent velocity, temperature, and curing (or equivalently, monomer concentration) profiles within the drop between the two cases.