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Size dependence of shape and stiffness of single sessile oil nano-droplets as measured by atomic force microscopy.

Munz, M; Mills, T* (2014) Size dependence of shape and stiffness of single sessile oil nano-droplets as measured by atomic force microscopy. Langmuir, 30 (15). pp. 4243-4252.

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Abstract

This article presents results and guidelines on the quantitative analysis of size, shape and stiffness of single sessile oil droplets in air and in water. Atomic force microscopy (AFM) facilitates the analysis of micro- and nano-scale droplets which are of growing importance for agrochemicals, cosmetics or foodstuffs containing emulsions with nano-scale compartments or droplets. Measurement of droplet shape and stiffness provides information on the contact angle with the support surface as well as the interfacial tension of the liquid-liquid interface. In this study, micro- and nano-scale droplets were imaged both in amplitude modulation (AM) and force mapping modes. The effects of the AM mode setpoint ratio on the measured droplet shape are discussed and a modified spherical cap model is suggested to extract the droplet-substrate contact angle. This model was applied to a population of different sized oil droplets imaged in water, and lead to the finding that the contact angle with the solid support varies with the droplet size. Force mapping was undertaken to measure the droplet stiffness as a function of the droplet size. Smaller droplets were found to be stiffer, in reasonable agreement with the Attard-Miklavcic model [Langmuir 17 (2001) 8217-8223] which describes the deformation of a sessile droplet in the non-wetting regime, i.e. by partial wrapping of the droplet around the probe surface. The model limitations are discussed in terms of the diverging droplet stiffness predicted for droplet radii similar to the probe radius as well as the error propagation associated with the droplet shape function.

Item Type: Article
Keywords: Oil-water emulsion, oil droplets, contact angle, interfacial tension, contact line tension, atomic force microscopy (AFM), amplitude modulation (AM), force mapping, measurement uncertainty
Subjects: Nanoscience
Nanoscience > Surface and Nanoanalysis
Nanoscience > Nano-Dimensional
Identification number/DOI: 10.1021/la5001446
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/6185

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