3.7. Atomic force microscopy (AFM)
AFM images provide detailed structure on morphology (qualitative parameter) and the roughness (quantitative parameter) ofthe biocomposite film surfaces. Fig. 4 shows surface morphologies and the corresponding results of roughness parameters (Ra and Rq)of the films. The oil-free film exhibits a relatively rough surface,with Ra and Rq values of 50.6–66.1 nm, respectively (Fig. 4a). The presence of OEO led to a marked decrease in the roughness of the films, as indicated by lower Ra and Rq values (4.46 and 9.08 nm,respectively, for 0.4% (w/v) essential oil) (Fig. 4b). These results are in agreement with those of Atarés et al. (2010) for soy protein isolate-based films containing cinnamon essential oil. These authors described that the liquid state of oils may provide the expansion of creamed droplets on the film surface after drying,which fill the irregularities of the film matrix surface and making it smoother. Nonetheless, as the oil content increased in the formulation, an increase of Ra and Rq was observed (Fig. 4c andd). This trend could be associated to the greater development of lipid aggregation and creaming during the drying step, and consequently producing irregularities on the film’s surface (Ghasemlouet al., 2013). The roughness differences between the control and OEO-containing films were accordant with the SEM microstructural observations.