Analysis of shape distortions in sessile drops

The effects of drop flattening and substrate surface heterogeneity on the cross-sectional and planar shapes of droplets of liquids resting on solid surfaces are examined. A simple method, volume single image sequencing, for examining a side view digital image profile is developed. Advantages of this sequential approach are that the influence of the finite pixel resolution is made explicit and the confidence with which shape deviations can be deduced can be quantified. A dual-camera video microscopy system was developed for simultaneously recording the planar and side views of sessile drops. Experimentally observed profiles have been analyzed for large and small drops of triply distilled water evaporating from poly(ethylene terephthalate) (Mylar), poly(methyl methacrylate), Teflon, and glass substrates. The planar view shows that deviations from a circular shape often occur in the latter stages of evaporation but that these are not apparent from a side view observation of the droplet. Experiments were conducted with drops of glycerine on a grooved glass substrate with side view observations both along and at 90° to the axis of the grooves. Two side views were also used with drops of glycerine on Mylar and Teflon, and this demonstrated the large variation in measured contact angle possible dependent upon the direction of the side view.