AN EXPERIMENTAL-STUDY OF THE GROWTH OF BREATH FIGURES

This paper describes an experimental study of the way in which a system of droplets (breath figures) evolves during the condensation of water vapour on a polyethylene film. The phenomenology observed is independent of the condensate-substrate contact angle assuming contact angle hysteresis is negligible. It is demonstrated, through the use of two experimental systems, that the droplets can grow according to two fundamentally different growth laws, a feature which has been the cause of an apparent anomaly between past studies. Despite the significant difference in the growth, both sets of results were found to be in good agreement with an empirical equation proposed by Vincent [R.A. Vincent, Proc. R. Soc. London, Ser. A, 321 (1971) 53.] which describes the time independent evolution of the process. The coalescence-free growth rate of the droplets was expressed in the form, dD/dt = kappa-D-beta where D is the droplet diameter and beta is a constant. The important period of coalescence growth, which occurs at the start of the condensation when the droplets are relatively isolated, characterises the first regime. The second regime is characterised by the period when coalescence has its maximum influence on the droplet growth. In the first experimental system the growth of the droplets was limited by the rate at which the latent heat could be dissipated. In this case kappa was a constant and the mean diameter of the droplets scaled as D(m) almost-equal-to t1/(1+beta) during both the first and second regimes. The value of beta was approximately 0.49. In the second experimental system the latent heat was easily dissipated and hence the system evolved according to a constant flux of condensing vapour. The parameter kappa varied with time accordingly. The result was that the mean diameter of the droplets scaled as D(m) almost-equal-to t1/3 during the first regime and scaled as D(m) almost-equal-to t during the second regime.