Methods
Bilayer Films
The bilayer creation method was adopted from the study done by Egunov AI et al in 2015. The bottom layer is created before the top layer. The bottom layer is pure PDMS. This was created by mixing PDMS (Sylgard 184) with its curing agent in a volume ratio of 10 : 1. The mixture is carefully poured onto a rectangular glass slide, and the mixture is leveled out with a wooden spatula. The glass slide is then placed in a 37°C oven for 20 minutes for curing. The top layer is created immediately after the curing of the bottom layer. This is done to prevent excess contamination on the bottom layer. Contamination may reduce the adhesion of the two layers when the top layer is applied. PDMS is created as explained previously. CrO2 powder solution is added to the PDMS in a 5:1 PDMS to CrO2 ratio. The top layer mixture also includes silicone oil (Rhodorsil, 47 V 350), at the desired concentration (v/v). The thickness of the top layer is weighed to be equal as the bottom layer, due to maximum curvature when the ratio between the two layers, m, is equal to one (Egunov AI et al, 2015). The cut pieces of film are placed in bottles with chloroform for 48 hours at room temperature. This process extracts the silicone oil from the top layer. After the oil extraction, the films are placed in a ventilated hood for another 48 hours to dry. This makes sure the chloroform has evaporated from the films.
CrO2 Powder
The CrO2 powder (480037, Sigma-Aldrich, USA) used in this project has an average particle size on the order of 10 μm (Li M et al, 2018). The powder is pretreated with sodium bisulfite using methods described in literature (Botter WJ et al, 1970). This process creates a reduced barrier surface layer. The treatment prevents oxidation reactions with water and organic solvents, allowing for the CrO2 powder to be directly mixed with PDMS without introducing any visible aggregation.
Curvature Measurement
A reference sheet was created to measure the curvatures of the films. Sheets with many circles of increasing radii in cm were designed. The films with spontaneous curvature were released from above a glass slide. This let them take their more favorable shape. The films lied on their thickness side, eliminating the effect of gravity on the curvature (Figure 1). They were matched to the circles that best fit their curvature profiles, and the measurements were recorded electronically.
Figure 1. Curvature measurement method. One film with spontaneous curvature is seen matched to its best fit circle of radius 0.89 cm.