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Interfacial Phenomena and Heat Transfer

ISSN Print: 2169-2785
ISSN Online: 2167-857X

Open Access

Interfacial Phenomena and Heat Transfer

DOI: 10.1615/InterfacPhenomHeatTransfer.2013007038
pages 29-49

DETERMINATION OF APPARENT CONTACT ANGLE AND SHAPE OF A STATIC PENDANT DROP ON A PHYSICALLY TEXTURED INCLINED SURFACE

Gaurav Bhutani
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Krishnamurthy Muralidhar
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India
Sameer Khandekar
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India

ABSTRACT

Estimating the apparent contact angle under equilibrium conditions is critical for the understanding of several engineering processes. Some examples are dropwise condensation, digital microfluidics, and material deposition schemes. Often, there is considerable uncertainty in the experimental estimation of the contact angle. In this work, we discuss the contact angles and shapes of pendant drops on physically textured inclined surfaces. Two methodologies to determine the apparent contact angles have been employed. In one approach these are obtained from drawing tangents at contact points of micro-droplets in optical images using digital image processing. In the second method, the three-dimensional (3D) Young–Laplace equation is numerically solved using the open-source software, Surface Evolver, by minimizing the sum of the potential and surface energies of the pendant droplet. A section of the numerically obtained 3D droplet shape is then fitted to the experimentally obtained two-dimensional profile using an inverse method. Advancing and receding angles of the imaged drop are calculated by minimizing the error between the numerical and experimental drop shapes, providing good estimates of these angles. In addition, the complete 3D droplet shape is also obtained. The overall methodology presented herein is generic, although the experiments have been conducted with glycerin as the working fluid. The role of surface roughness, plate inclination, and drop volume on the advancing and receding angles of a pendant drop are discussed. On inclined surfaces, the three-phase contact line does not remain pinned and its shape is not circular. The receding angle progressively diminishes with inclination while the advancing angle remains nearly constant.