The phenomenon of thermocapillarity, the response of fluids to thermal gradients due to thermal alteration of their surface tension, was first reported over a century ago. Only in the past two decades has the deliberate use of thermocapillary forces for the patterning of soft matter been actively pursued, either for the ordering of internal structure or the introduction of topographic features. The application of focused laser spike (FLaSk) excitation can generate gradients up to ~1000 K/μm at a microscale spot. These extreme thermal gradients produce thermocapillary response with a high degree of spatiotemporal control and allow, through local softening, for the use of samples that are solid both before and after the excitation, locking in the effects of the exposure. This is the key advantage of the approach - thermal gradients simultaneously provide both the mobility and driving force for assembly. The soft-shear of moderate thermal gradients leads to the simultaneous annealing and alignment of mesoscale structure, while higher gradients lead to complete dewetting of films into trench-ridge patterns with overlap effects. The forces involved are great enough to overcome surface affinity and lead to morphological development defined by the fluid nature of the molten regions of polymers and even metals. During this processing, additional effects such as multilayer mobility, overlap, and simultaneous physical vapor deposition result in the generation of hierarchical micro/nanostructures. As a means of lithography, FLaSk possesses the advantages of being non-contact, subwavelength, developer and photochemical-free, and applicable to a wide array of materials.
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