Shape-Shifting Droplet Networks

Collaborators

Tao Zhang, D. Wan, J. M. Schwarz, and M. J. Bowick$^{*}$

Soft Matter Program and Department of Physics, Syracuse University, Syracuse, NewYork 13244, USA
$^{*}$mjbowick@syr.edu

Motivation

Biological self-assembly, such as the folding of polypeptide chains into proteins, is the process by which smaller components spontaneously organize into ordered structures. Taking a cue from biology, researchers have engineered materials that, through self-assembly, fold into designated geometries. Recent work, for instance, showed that sheets of aqueous droplets can assemble into a variety of three-dimensional shapes. Expanding on this result, we have now demonstrated theoretically that such droplet networks can be programmed to reversibly switch between different shapes. This finding is a step toward biologically inspired robots that can change their shape according to their environment.

Schematic image showing droplets of different aqueous solutions printed into a solution of lipids in oil. The droplets acquire a lipid monolayer and form bilayers with droplets in the developing network.
Schematic image showing droplets of different aqueous solutions printed into a solution of lipids in oil. The droplets acquire a lipid monolayer and form bilayers with droplets in the developing network.

Approach

We modeled sheets of micrometer-sized water droplets joined by permeable single lipid bilayers to form a tissue-like structure. By varying the concentrations of solutes within the droplets, the researchers created osmotic pressure that, by swelling some of the droplets and shrinking others, can cause the sheets to fold into several possible structures. The researchers focused on a configuration investigated in previous experiments—a four-petal design that spontaneously folded to produce a hollow sphere. They then demonstrated that they could reverse the shape change by placing the hollow sphere in a liquid medium with a higher solute concentration. According to their calculations, the droplets in the sphere lost water and shrank, leading the sphere to unfold back into the flat four-petal shape.

(a)–(c) Three snapshots from the reversible folding process. (d) Reversibility dependence on the z coordinate of the horizontal plane and the osmolarity of the surrounding medium.
(a)–(c) Three snapshots from the reversible folding process. (d) Reversibility dependence on the z coordinate of the horizontal plane and the osmolarity of the surrounding medium.

Acknowledgments

The authors acknowledge useful comments by M. C. Marchetti on an earlier draft of the manuscript. M. B. thanks L. Mahadevan for stimulating discussions. This research was supported by the Soft Matter Program at Syracuse University.

Tao Zhang
Tao Zhang
Special Research Fellow

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