Splash-wrapping of droplets in Science

A polystyrene sheet that is 1,000 times thinner than a human hair is impacted by a falling oil droplet. The end result is droplet encapsulated by a flexible sheet.

Thin elastic sheets make surprisingly good wrappers for liquid droplets: surface tension will spontaneously pull an ultrathin sheet around a droplet, all while making efficient use of the sheet. The wrapper can be used for a variety of tasks: it provides a strong barrier for protecting the liquid cargo, it can deform the droplet into predictable shapes, and it provides a platform for adding a chemical pattern. But creating many such droplets requires a rapid and scalable process. A new technique uses droplet impact on a floating polymer film to achieve a tidy wrapping in a fraction of a second. The experiments were carried out by Deepak Kumar and Joseph Paulsen at UMass Amherst, and the results are published here.

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Lab YouTube channel

A new Paulsen Lab YouTube channel features video clips of liquid droplets and thin elastic sheets. Current content is from recent work done at UMass Amherst and UChicago, plus an interview by the Syracuse University College of Arts & Sciences. More clips coming soon!

Geometry-driven folding in PRL

A thin polymer film with an annular shape that is floating on water. As the surface tension pulling on the outer edge is lowered, the sheet forms wrinkles and then two folds. The sheet is 394 nm thick and 16 mm wide.

Wrinkles are all around us — on hanging curtains, the skin of drying fruit, or a surprised forehead. The more a material is squished, the deeper and taller the wrinkles become, until they collapse into a fold. Typically, this process depends strongly on the materials in question, for example the thickness of the skin, or the softness of the flesh underneath. However, we show that a wrinkle-to-fold transition may be affected only by the shape of the compressed object, rather than by any mechanical properties!

New postdoc

The group welcomes postdoctoral researcher Yousra Timounay. Yousra comes from the groups of Florence Rouyer and Elise Lorenceau at the Navier Laboratory in Paris, France. Welcome, Yousra!

CAREER award

The Paulsen Group has received a CAREER award from the National Science Foundation for a project titled: “Ultrathin sheets on curved liquid surfaces: Stress focusing and interfacial assembly”.

Grant from American Chemical Society

The Paulsen Group was recently awarded a “Doctoral New Investigator” grant from the ACS Petroleum Research Fund, for a project titled: “Hyperuniform Dispersal of Non-Brownian Particles in Viscous Liquids”. See a video profile here.

Curved wrinkles in the news

A thin polymer film floating on water, poked from beneath. As seen in profile (bottom), the overall shape of the sheet is curved. The wavelengths of the wrinkles are strongly influenced by the local curvature.

Recent work on wrinkles in curved topographies was highlighted in a UMass Amherst press release, which was picked up here and here.

Wrinkling on curved surfaces in PNAS

Pictured is a thin polymer film floating on a bath of water and draped over a metal sphere, giving rise to radial wrinkles. The sheet is 157 nm thick and 22.2 mm wide.

“Curvature-induced stiffness and the spatial variation of wavelength in wrinkled sheets” appeared in PNAS (link). This work proposes and experimentally tests a basic framework for predicting wrinkle wavelengths in curved and/or non-uniform topographies. 

Cover of Nature Materials

The cover of the December 2015 issue of Nature Materials shows a partially encapsulated water drop in oil, from “Optimal wrapping of liquid droplets with ultrathin sheets” (link).