NCAFM2023 Programme Booklet

NANOSCALE WETTING USING NON-CONTACT ATOMIC FORCE MICROSCOPY TECHNIQUES: COMPLEXITY ON THE NANOSCALE

Jaime Colchero

Centro de Investigación en Óptica y Nanofísica (CIOyN), Departamento de Física, Universidad de Murcia, Campus Espinardo, E-30100 Murcia Email: colchero@um.es

Understanding of wetting properties of surfaces is still a formidable challenge, and it is far from clear how nanoscale chemistry and morphology determine macroscopic wetting. In this field Nature seems still ahead of first principles calculations and theory; if surfaces with well-defined and robust wetting properties are to be designed, it is still more effective to “look” at what Nature offers, than to rely on theoretical modeling. The Lotus and the Rose Petal effect are two striking examples of how Nature fine-tunes nanoscale chemistry and physics to obtain an appropriate function on the macro-scale. In the present work, we use Dynamic Atomic Force Microscopy (DAFM) to simultaneously measure the nanoscale morphology and chemistry of Rose Petals [1]. Due to the very high roughness, and the fact that a detailed analysis of surface roughness is required, 3D modes (such as “Jumping Mode” or “Force Volume”) cannot be used since they are intrinsically slow [2]. We show that using appropriate imaging parameters, the local wetting properties can be measured using non-contact Dynamic Atomic Force Microscopy in humid air. As an application of this scheme, we investigate the surfaces of Rose Petals finding two extraordinary features linked to their peculiar wetting properties: (i) surface roughness is concentrated on the nanoscale and fractal-like, and (ii) the surface has an extreme nanoscale chemical variability. While high roughness is generally accepted to be the origin of super-hydrophobicity the role of nanoscale chemical variability is usually not a topic of discussion; most probably because -up to now- it could not be “seen”. From our data, we conclude that a liquid drop will strongly adhere to nanoscale hydrophilic patches and detach from all the other hydrophobic parts of the surface. The combined effect of roughness and chemical (nanoscale) wetting properties will induce a high effective nanoscale contact angle, which explains the surprising and in principle antagonistic properties associated with the Rose Petal Effect: high contact angle and high drop adhesion. Although this work is focused on rose petals, the fundamental mechanism by which Nature is able to generate high contact angle and drop adhesion will apply to other surfaces, and non-contact DAFM seems and ideal tool to study nanoscle wetting.

Fig. AFM images showing topography (vertical scale, 3d representation) and wetting properties coded as the color of surface. blue: hydrophilic, red: hydrophobic. Note that the more hydrophilic regions tend to be at the highest regions of the surface. Lateral scale: 12.5 mm, vertical z-scale (height) about 4 mm.

References [1] L. Almonte, et. al., 2021, Nano Select, 3 (5), 97–989. [2] J.F. Gonzalez-Martinez, et. al., 2012, Nanoscale Research Letters 7(1):174.

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