Effects of Impact Inertia and Surface Characteristics on Deposited Polymer Droplets in Microcavities

Microflow visualization and computational fluid dynamics (CFD) are complementarily performed to study the evolution of a single poly(ethylenedioxythiophene) (PEDOT) droplet ejected from a piezoelectric ink-jet printhead and the equilibrium film characteristic of the droplet deposition in a microfabricated cavity. The verified CFD code is further applied to investigate the influences of contact angles $\theta_{\rm s}$ of the PEDOT droplet/air interface and the PEDOT droplet/cavity sidewall interface as well as droplet impact velocity $V_{\rm d}$ on the transient deposition process in the microcavity. Impact inertia was studied by varying the droplet Weber number from 30.3 to 42.6. The surface characteristics are explored by choosing $\theta_{\rm s}$ of 10$^{\circ}$, 30 $^{\circ}$ , 50$^{\circ}$ , 70$^{\circ}$, 90$^{\circ}$, and 110 $^{\circ}$. The influences of impact inertia are also examined by increasing $V_{\rm d}$ from 2.0 to 12.0 m/s at 2.0 m/s interval. The computed results are found in good agreement with the experimental ones. For the first time, critical Weber numbers have been found relating to the ability of the droplet to wet the side walls and fill a microcavity with a uniform film. The results are also new in terms of the identifications of the critical contact angle $(\theta_{\rm s})_{\rm c}$ and critical impact velocity $(V_{\rm d})_{\rm c}$. At $( \theta_{\rm s})_{\rm c}$ and at and beyond $(V_{\rm d})_{\rm c}$ , the formation of an intact flat film in the cavity is fulfilled. $\hfill$ [2007-0226]