Thermally induced interfacial instabilities and pattern formation in confined liquid nanofilms

The dynamics, instability, and pattern formation of thermally triggered thin liquid films are investigated numerically under a long-wave limit approximation. To determine the mechanisms responsible for instability growth and pattern formation in confined heated nanofilms, acoustic phonon (AP) and thermocapillary (TC) models are examined using both linear and nonlinear analyses. Under uniform heating conditions, both AP and TC models predict the formation of raised columnar structures (pillars) and bicontinuous structures for very low and high filling ratios defined as the ratio of the initial film thickness to the plate separation distance, D-1. A transition threshold is observed when D approximate to 2.5. However, the TC model predicts smaller features for larger D, whereas the opposite prediction applies for the AP model. Under spatially variable cooling conditions involving a patterned top plate, both TC and AP models exhibit similar predictions: pillars form under the top plate protrusions. When the heating is spatially variable, the lower plate is patterned; the AP model predicts pillar formation above ridges, whereas the TC model predicts pillar formation above the valleys.