Physics - Applied Physics and Physics - Fluid Dynamics
Abstract
Current methods for fabricating lenses rely on mechanical processing of the lens or mold, such as grinding, machining, and polishing. The complexity of these fabrication processes and the required specialized equipment prohibit rapid prototyping of optical components. This work presents a simple method, based on free-energy minimization of liquid volumes, which allows to quickly shape curable liquids into a wide range of spherical and aspherical optical components, without the need for any mechanical processing. After the desired shape is obtained, the liquid can be cured to produce a solid object with nanometric surface quality. We provide a theoretical model that accurately predicts the shape of the optical components, and demonstrate rapid fabrication of all types of spherical lenses (convex, concave, meniscus), cylindrical lenses, bifocal lenses, toroidal lenses, doublet lenses and aspheric lenses. The method is inexpensive and can be implemented using a variety of curable liquids with different optical and mechanical properties. In addition, the method is scale-invariant and can be used to produce even very large optical components, without a significant increase in fabrication time. We believe that the ability to easily and rapidly create high-quality optics, without the need for complex and expensive infrastructure, will provide researchers with new affordable tools for fabricating and testing optical designs.
We present a theoretical description and an experimental realization of a thermocapillary dipole induced in a Hele-Shaw cell under a steady temperature gradient. We demonstrate experimentally how several dipoles can be superposed in order to create various 2D flow patterns, and how a confined dipole can act as a thermocapillary motor for driving fluids in microfluidic circuits. In addition, we show how the principles behind the thermocapillary dipole can be applied in order to drive thermocapillary swimmers on fluid-liquid interfaces.
Rofman B, Dehe S, Frumkin V, Hardt S, and Bercovici M
Langmuir : the ACS journal of surfaces and colloids [Langmuir] 2020 May 26; Vol. 36 (20), pp. 5517-5523. Date of Electronic Publication: 2020 May 12.
Abstract
Wetting transition on superhydrophobic surfaces is commonly described as an abrupt jump between two stable states-either from Cassie to Wenzel for nonhierarchical surfaces or from Cassie to nano-Cassie on hierarchical surfaces. We here experimentally study the electrowetting of hierarchical superhydrophobic surfaces composed of multiple length scales by imaging the light reflections from the gas-liquid interface. We present the existence of a continuous set of intermediate states of wetting through which the gas-liquid interface transitions under a continuously increasing external forcing. This transition is partially reversible and is limited only by localized Cassie to Wenzel transitions at nanodefects in the structure. In addition, we show that even a surface containing many localized wetted regions can still exhibit extremely low contact angle hysteresis, thus remaining useful for many heat transfer and self-cleaning applications. Expanding the classical definition of the Cassie state in the context of hierarchical surfaces, from a single state to a continuum of metastable states ranging from the centimeter to the nanometer scale, is important for a better description of the slip properties of superhydrophobic surfaces and provides new considerations for their effective design.
