Thin Film Rheology via Focused Laser Spike Thermocapillary Dewetting

Focused laser spike (FLaSk) dewetting employs the localized heat source of a focused laser to create thermocapillary-induced trench-ridge morphologies. By using a universal heating substrate to create a material-independent thermal profile coupled with optical microscopy, we have studied the dewetted ridge feature for several distinct glassy thin films. The evolution of the ridge's radius over time can be modeled using stretched exponential functions to derive a maximum dewetted radius and a characteristic decay time. The characteristic decay time shows a super-Arrhenius behavior resembling viscosity change during the glass transition process. The extracted activation energies are independent of thickness and show direct correlation with molecular weight and entanglement. Further, an effective viscosity of TC dewetting may be defined that correlates with previous theoretical and experimental results. The extracted values appear to also capture additional ordering mechanisms, as indicated by the abnormally high activation energy and resultant FLaSk viscosity of the molecular glass N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine. In this way, we have demonstrated that FLaSk rheology can be a useful tool to probe difficult to investigate regimes of thin film fluid mechanics that can be developed through more systematic study.