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Partic. vol. 33 pp. 91-97 (August 2017)
doi: 10.1016/j.partic.2016.08.005

Analysis of humidity-dependent adhesion between a probe tip and a surface

Li Yang, Junhui Hu*, Huapeng Xiao, Wenwen Quan

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hujh@mailbox.gxnu.edu.cn

Highlights

    • A numerical method to calculate the adhesion force between tip and surface was proposed. • The shape of the probe tip was denoted by a power-law profile including a shape index. • A procedure to calculate van der Waals force in the presence of the liquid bridge was presented. • The relationship between the liquid bridge radius and the half-filling angle was derived. • We identified the objects’ shape as the origin of different adhesive force vs. humidity behaviors.

Abstract

Adhesive forces commonly exhibit a monotonic increase or a maximum with increasing relative humidity. However, anomalous behavior has been reported. Here, a numerical model of adhesive forces, comprised mainly of capillary and van der Waals forces, between a tip and a surface is established. It is described by a power law that considers the geometry, the liquid bridge wetting radius, the contact angle, and the separation distance. Capillary forces (sum of surface tension and Laplace pressure) and van der Waals forces are calculated. The latter cannot be neglected in the adhesion even at high humidity. Decrease in adhesion with increasing relative humidity can be attributed to a blunt tip shape, which is validated by experimental data. Specifically, the decrease in adhesion is attributed primarily to a transition from a rounded to a blunt tip shape. Structuring objects at the micro- or nanoscale can either increase or decrease adhesion as a function of relative humidity. This has a wide range of applications in robotic manipulation and can provide a better understanding of adhesion mechanisms in atomic force microscopy in ambient air.

Graphical abstract

Keywords

Abnormality decrease; Adhesive force; Tip shape; Capillary force; Van der Waals force