Dropwise Condensation Studies on Multiple Scales

Abstract

Recent advances in nanotechnology, chemical/physical texturing and thin film coating technology generate definite possibilities for sustaining a dropwise mode of condensation for much longer durations than was previously possible. The availability of superior experimental techniques also leads to deeper understanding of the process parameters controlling the relevant transport phenomena, the distinguishing feature of which is the involvement of a hierarchy of length/time scales, proceeding from nuclei formation, to clusters, all the way to macroscopic droplet ensemble, drop coalescence, and subsequent dynamics. This paper is an attempt to connect and present a holistic framework of modeling and studying dropwise condensation at these multiple scales. After a review of the literature, discussions on the following problems are presented: (i) atomistic modeling of nucleation; (ii) droplet–substrate interaction; (iii) surface preparation; (iv) simulation of fluid motion inside sliding drops; (v) experimental determination of the local/ average heat transfer coefficient; and (vi) a macroscopic model of the complete dropwise condensation process underneath horizontal and inclined surfaces. The study indicates that hierarchal modeling is indeed the way forward to capture the complete process dynamics. The microscopic phenomena at the three-phase contact line, leading to the apparent droplet contact angle, influence the shear stress and heat transfer. The nucleation theory captures the quasi-steady-state behavior quite satisfactorily, although the early atomistic nucleation was not seen to have a profound bearing on the steady-state behavior. The latter is strongly governed by the coalescence dynamics. Visual observation of dropwise condensation provides important information for building hierarchical models.

Acknowledgments

The authors are grateful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Government of India, for providing the partial financial assistance to carry out this research work. Technical discussions with Dr. L. M. Gantayet and Dr. Jaya Mukherkjee, Bhabha Atomic Research Center, Mumbai, India, are gratefully acknowledged. Contributions by former students, Gagan Bansal, Gaurav Bhutani, Nirmal Kumar Battoo, and Liza Ann Easo, who worked on the project, and technical help provided by C. S. Goswami are also acknowledged. The authors are grateful to the anonymous referee who examined the first version of the paper very critically and provided useful suggestions.

Notes

Condensing vapor that is supercooled below its equilibrium saturation temperature and condensing liquid that is superheated with respect to its equilibrium saturation temperature can prevail for short periods of time and are referred to be in a metastable state.

Unless otherwise stated, in this paper we always deal with the apparent contact angle. The molecular contact angle formed by the precursor layer existing at the three-phase contact line is not considered in this discussion [31].

Heat transfer during dropwise condensation depends strongly on the surface properties and surface phenomena, especially on contact angle. To understand this effect, Neumann et al. [45] performed experiments for a coated substrate. Each run was of 300 min duration. The contact angle for horizontal substrate and contact-angle hysteresis for inclined substrate were measured before each run. It was found that after each run the heat flux decreased considerably. The reduction was caused by a deterioration of the condenser surface, which in turn increases the contact-angle hysteresis and size of droplet at slide-off or fall-off. If surface properties remained constant, there would be no change in the heat flux of each run.

The actually observed critical drop radius during the experiment by Sikarwar et al. [46] was slightly higher than that predicted by Eq. (41). Factors such as local droplet pinning and physicochemical inhomogeneities of the substrate, which are not considered in Eq. (41), are believed to be responsible for this discrepancy.