Figure 4. Accuracy of FreSCa cryo-SEM contact angle measurements.

(a) Error in contact angle measurements σ_θ by FreSCa calculated according to equations (3) and (4) as a function of k for fixed k/l=0.1 for shadowing angles α=30° (blue), 45° (green) and 60° (red), corresponding, respectively, to contact angles of 46°, 67° and 85°. Any other choice of k/l leads to the same qualitative behaviour. σ_θ decreases for increasing k and with smaller shadowing angles. (b) σ_θ as a function of r for a fixed contact angle θ=110° for shadowing angles α=30° (blue), 45° (green) and 60° (red). The accuracy is increased by reducing α and for larger particles. Any other choice of θ leads to equivalent results. (c) Cryo-SEM image of a 500 nm amidine latex nanoparticle (NP) at the frozen water/n-decane interface. The sample was coated three times in succession with 3 nm of tungsten, each time at a different shadowing angle to produce shadows of different lengths as highlighted on the image: α=30° (blue), α=45° (green) and α=60° (red). The graininess of the background and of the particle surface is due to the thick metal layer from to the triple coating. Inset: cryo-SEM image of two neighbouring 200 nm amidine latex NPs at the frozen water/n-decane interface. Despite having the same size, the two particles show significantly different contact angles (left particle r=116.4±2.5 nm θ=96.9±3.13°; right particle r=116.3±2.5 nm θ=102.0±3.24°), highlighting the heterogeneity of wetting on the nanoscale. Scale bars, 200 nm.