Fundamentals of Thermocapillary Sculpting of Liquid Nanofilms and Applications to Thin Film Micro-Optics

This doctoral thesis describes experimental work conducted as part of ongoing efforts to identify and understand the source of linear instability in ultrathin liquid films subject to large variations in surface temperature along the air/liquid interface. Previous theoretical efforts by various groups have identified three possible physical mechanisms for instability, including an induced surface charge model, an acoustic phonon model, and a thermocapillary model. The observed instability manifests as the spontaneous formation of arrays of nano/microscale liquid protrusions arising from an initially flat nanofilm, whose organization is characterized by a distinct in-plane wavelength and associated out-of-plane growth rate. Although long range order is somewhat difficult to achieve due to thin film defects incurred during preparation, the instability tends toward hexagonal symmetry within periodic domains achieved for a geometry in which the nanofilm is held in close proximity to a cooled, proximate, parallel, and featureless substrate. In this work, data obtained from a previous experimental setup is analyzed and it is shown how key improvements in image processing and analysis, coupled with more accurate finite element simulations of thermal profiles, lead to more accurate identification of the fastest growing unstable mode at early times. This fastest growing mode is governed by linear instability and exponential growth. This work was followed by re-examination of real time interference fringes using differential colorimetry to quantify the actual rate of growth of the fastest growing peaks within the protrusion arrays. These initial studies and lingering questions led to the introduction of a new and improved experimental setup, which was redesigned to yield larger and more reproducible data sets. Corresponding improvements to the image analysis process allowed for the measurement of both the wavelength and growth rate of the fastest growing mode simultaneously. These combined efforts establish that the dominant source of instability is attributable to large thermocapillary stresses. For the geometry in which the nanofilm surface is held in close proximity to a cooled and parallel substrate, the instability leads to a runaway process, characterized by exponential growth, in which the film is attracted to the cooled target until contact is achieved. The second part of this thesis describes fabrication and characterization of microlens arrays and linear waveguide structures using a similar experimental setup. However, instead of relying on the native instability observed, formation and growth of liquid shapes and protrusions is triggered by pre-patterning the cooled substrate with a desired mask for replication. These preformed cooled patterns, held in close proximity to an initially flat liquid nanofilm, induce a strong non-linear response via consequent patterned thermocapillary stresses imposed along the air/liquid interface. Once the desired film shapes are achieved, the transverse thermal gradient is removed and the micro-optical components are affixed in place naturally by the resultant rapid solidification. The use of polymer nanofilms with low glass transition temperatures, such as polystyrene, facilitated rapid solidification, while providing good optical response. Surface characterization of the resulting micro-optical components was accomplished by scanning white light interferometry, which evidences formation of ultrasmooth surfaces ideal for optical applications. Finally, linear waveguides were created by this thermocapillary sculpting technique and their optical performance characterized. In conclusion, these measurements highlight the true source of instability in this geometry, and the fabrication demonstrations pave the way for harnessing this knowledge for the design and creation of novel micro-optical devices.