Shape memory materials are smart materials that can recover their original shapes from a plastic deformation when an external stimulus is exerted. In the past decades, a number of shape memory materials such as shape memory alloys and shape memory polymers have been successfully developed. However, it remains a challenge to develop advanced shape memory materials with more functions and better performance to meet the growing demands for complicated engineering applications. Specially, liquid-based shape memory material, endowed with intrinsic fluidity and flexibility, are still scarce. In this study, we report a novel design of liquid-based shape memory material – liquid nanofoam (LN), employing the reversible liquid flow behavior in hydrophobic nanopores. The LN material is composed of a hydrophobic nanoporous media and a nonwetting liquid phase. At sufficiently high pressure, the liquid is driven into the hydrophobic nanopores. As the pressure is removed, the infiltrated liquid can fully or partially flow out from the nanopores. It is hypothesized that the system volume of LN can be precisely controlled by tuning the extent of liquid outflow from nanopore thermally.The shape memory performance of LN material is evaluated by quasi-static cyclic compression tests at a loading speed of 2 mm/min using an Instron universal tester. Firstly, at T = -60 ºC, the system shows a plastic deformation of -19.9% after the first loading-unloading cycle, indicating that the infiltrated liquid is “locked” in the hydrophobic nanopores. As the temperature gradually increase to T = 25 ºC, the infiltrated liquid flows out immediately, and the system volume is fully recovered to its original value. Thus, the LN system shows an instant shape memory behavior when subjected to heating. Secondly, the fully recovered LN system is cooled down to T = -60 ºC. The system volume shows negligible change since the liquid cannot infiltrate into the nanopores spontaneously due to the energy barrier of the hydrophobic surface. As another loading-unloading cycle is applied, the system volume exhibits a -19.9% reduction again, revealing the liquid is infiltrated and later locked in the nanopores. Therefore, the LN system shows a fully reversible but delayed shape memory response when the system temperature is decreased. This unique bidirectional shape memory response is related to the thermoresponsive bubble nucleation process in the nanoconfined environment. A phenomenological constitutive model has also been developed to describe this thermoresponsive shape memory behavior. These findings open up new avenues for the development of novel intelligent systems.

A Shape Memory Material Enabled by Reversible Liquid Flow in Hydrophobic Nanopores

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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