Abstract
The structuring of liquid–liquid and liquid–air interfaces may play an important role in novel microfabrication platforms and biotechnologies, from the spontaneous formation of microfilaments from liquid droplets and the 3D printing of liquids, to the culture of stem cells on emulsions. Understanding the mechanical anisotropy of associated liquid interfaces is essential for the development of such systems. Models of AFM indentation at liquid interfaces, based on the Young-Laplace model, currently do not allow the quantification of interfacial mechanical properties of associated molecular films. This report presents such a model and compares its predictions to interfacial mechanical properties characterised via interfacial shear rheology. An extreme reversal of mechanical anisotropy of liquid–liquid interfaces is observed, upon self-assembly of protein nanosheets, by 5 orders of magnitude. Results indicate that, although interfacial rheology is more sensitive than AFM indentation to the mechanics of molecular films in the low range of interfacial mechanics, AFM indentation allows the quantification of mechanical properties of stiffer molecular films, and remains better adapted to the characterisation of small samples and enables the characterisation of local heterogeneity.
Highlights
The structuring of liquid interfaces plays an important role in many processes and technological platforms, from the modulation of surface tension at the air-liquid interface by lung surfactants [1], the stabilisation of foams by natural surfactant proteins of the Ranaspumin family, to protect frog eggs [2], and the mechanical strengthening of liquid interfaces for the protection of biofilms [3, 4], to the printing of liquid formulations [5] and the engineering of microdroplets [6]
Comparison of shear and indentation moduli of liquid-liquid interfaces To characterise the mechanical properties of nanosheets assembled at the surface of oil droplets, we proposed to extend a model describing the approach of a rigid probe deforming a liquid interface (Figure 1) [11, 12, 14, 15]
It is worth pointing out that the notion of surface tension in these systems, with protein nanosheets adsorbed at interfaces, is not fully appropriate, as the naked liquid-liquid interface has been replaced with a film, with limited further dynamic exchange with surrounding media
Summary
The structuring of liquid interfaces plays an important role in many processes and technological platforms, from the modulation of surface tension at the air-liquid interface by lung surfactants [1], the stabilisation of foams by natural surfactant proteins of the Ranaspumin family, to protect frog eggs [2], and the mechanical strengthening of liquid interfaces for the protection of biofilms [3, 4], to the printing of liquid formulations [5] and the engineering of microdroplets [6]. AFM indentation has not yet been widely applied for such characterisation, it is attractive as it allows to gain local information on interfacial mechanics, and allows the study of interfaces with relatively low surface areas and volumes (e.g. microdroplets). To this aim, models enabling the dissociation of the contributions of interfacial interaction forces from the surface tension have allowed the quantitative description of AFM indentation profiles [11, 12]. Models enabling the extraction of quantitative mechanical properties of molecular films underpinning the structuring of liquid interfaces have not been developed
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