Experiment Designed by D. Quere

A teflon tube (cf. Fig. 1.20) of inner diameter 0.5 mm is connected to a syringe filled with air and the latter is placed on a motorised syringe plunger. A droplet of an alkane (nonane, dodecane or hexadecane) is deposited at the end of the tube. The syringe piston moves forward steadily and the air pushes the droplet along at constant speed. Speeds between 10 pm/s and 1 cm/s can be obtained in this way. 

The alkane partially wets the teflon. For each alkane, measure the advancing and receding angles by examining the capillary rise. 

The tube is transparent and the droplet can be photographed. By comparing two photos of the droplet, taken at 50 cm intervals, we conclude that, if the volume Q remains constant, no film has been deposited but if O should diminish, we conclude that the droplet has left a film of liquid behind it, of measurable thickness. 

The aim of the experiment is two-fold firstly, to reveal and determine the value of the threshold speed Kid beyond which a film will be dragged along secondly, to study what happens above this threshold V Kn), and examine the law describing how the thickness of the deposited film varies with speed. 

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