Towards nm scale continuous thin film of liquid metals for stretchable electronics
Flexible and stretchable electrodes and sensors have a rising demand in soft robotics, wearable devices, and electronic displays. Liquid metals are used in combination with polymers to synthesise soft conducting materials widely. Liquid metals such as Gallium (Ga), Eutectic Gallium Indium (EGaIn) and Galinstan inherently have high surface tension which inhibits the spreading on a substrate. Several works have been carried out to spread Ga on a surface, which require processing in micron-size thickness, multiple steps and involving different chemical reactions. In this work, we study a simple process of exposing the Styrene-ethylenebutylene-styrene (SEBS) substrate to plasma and deposit Ga of around 300 nm thickness through thermal evaporation to attain a soft conducting material.
Surface energies of the substrate plays a vital role in the spreading of liquid metals and thereby influencing the formation of thin films which are conducting. When pristine SEBS is deposited with Ga, small spherical droplets of Ga of ~50 nm diameter are observed, with a lesser contact in between them due to spherical nature. Non-spreading of the Ga droplets can be attributed to the low surface energy of the SEBS (~10 N/mm) and high surface energy of Ga (~700 N/mm). Plasma treatment of the substrate increases its surface energy by creating -OH groups on the surface. When any liquid of high surface energy is dispensed on the plasma treated surface, it tries to reduce its surface energy by increasing the spreading of liquid. Distinctly spread droplets of Ga can be observed for a plasma treated SEBS , which aids the tunnelling conductivity to occur and resulting in a conducting surface with resistance in the order of 10 KΩ. The spreading of Ga droplets can also be visualised from the surface forces acting at the three-phase contact line of the droplet
. Contact angle θ gets reduced, when the system tries to minimize the surface energy of the substrate. Encapsulation of the Ga layer helps in two ways. Nature of Ga to form oxide layer when exposed to atmosphere affects the durability of the substrate with decreased conductivity, which is prevented with an encapsulation layer. The other important advantage of an encapsulation layer is it helps to get a stretchable electrode with the least drop in resistance upon stretching up to 50%. Even though Ga is in liquid state at room temperature, the stretchability is limited due to the pinning of the three-phase contact line of droplets. SEBS-Ga layer without encapsulation tends to form distinct islands with a very less contact up on stretching to 25 % strain. The formation of cracks in the Ga layer increases the change in resistance with strain, thereby limiting its applications in stretchable electrodes but opening applications in stretchable strain sensors. Encapsulated samples were subjected to 50% strain and measurements were taken in parallel and perpendicular directions with respect to the stretching direction for 100 and 50 cycles respectively. The stability of the devices is promising to build resistive strain sensors which can find applications in soft robotics and wearable devices.
1. Thin Film instabilities
a. Thin film dewetting on patterned substrates for dynamic photonic devices.
Stretchable photonics is a subject of intense research, having huge potential for applications in soft robotics, biosensors, solar photovoltaics, mechanochromic sensors, wearable photonics, fast communication and computing. Compared to inorganic materials, organic polymers are inexpensive, easy to process and flexible enough to meet a broad range of application specific requirements. However, their applications are limited due to their low refractive index. We rely on thermal dewetting of chalcogenide glasses on patterned soft substrates to form stretchable metasurfaces
b. Understanding instability dynamics of thin film on soft substrates.
Dewetting of thin films, a process in which metastable or unstable films break up by one or combination of mechanisms such as amplification of thermal fluctuations in film thickness (spinodal dewetting), nucleation of holes through thermal annealing and heterogeneous nucleation to form nano-sized droplets. Thermal annealing and heterogenous nucleation results in random positioning of the droplets. Whereas isotropic arrangement of droplets is attained through spinodal dewetting which has a dominant characteristic wavelength in between the droplets. However, the formation of dewetted droplets in 1D, 2D or hexagonal arrangements on a flat substrate has remained challenging except for a few studies. Higgins et al has shown by inducing roughness on the substrate well defined one-dimensional ordering of the polymer film could be attained. On the whole patterning of nanodroplets through a simple dewetting process would be a significant advancement for a large area based applications.
In this work we try to understand the instability dynamics of thin films on soft substrates which could potentially be exploited to create 1D and 2D ordered patterns. The difference in the coefficient of thermal expansion between soft substrate and film leads to in-plane stresses in the film resulting in cracks. By controlling the periodicity of the cracks through substrate softness and film thickness, we control the droplets size and periodicity of the nanostructure.