Can lenses be shaped with just a hotplate? The secret lies in the understanding of dynamic surface tension of a fast curing liquid droplet. Sung and co-workers explored this phenomenon by depositing a small droplet of polydimethylsiloxane (PDMS) onto a hotplate. The authors observed that, at higher temperatures, a fast curing PDMS liquid droplet curved in a highly dynamic fashion which is not well-defined by the standard Laplace-Young equation. Through a series of numerical modelling using the freely available Surface Evolver (an interactive program for the modelling of liquid surfaces) and side-on optical imaging, they proposed that the surface tension of a fast curing droplet in a hotplate is akin to a normal liquid droplet with temperature-dependent surface tension, termed as effective surface tension (EST). From the new model, they demonstrated that a curing PDMS droplet increases its EST. This is in contrast with heated non-curing liquid droplets where surface tension decreases. The effects of EST consequently result in millimeter-sized droplet lenses with different curvatures/focal lengths. This work has the potential to open up new “thermal” free-form shaping techniques for fabricating PDMS optical elements that are found to be useful in applications such as microscopy and resonators.
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