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XC. A Problem in Viscosity : The thickness of liquid 

 films formed on solid surfaces under dynamic conditions. 

 By The Research Staff of the General Electric Company 

 Ltd., London. (Work conducted by F. S. GrOUCHER and 

 H. VYard.) * 



[Plate VIII.] 



Summm 



y- 



TF1HE problem of determining the thickness of the liquid 

 JL layer coating a solid body drawn out of a liquid is 

 discussed theoretically and practically. It is shown that if 

 the solid is a flat slab of infinite width, the forces deter- 

 mining the thickness are those of gravity (g) and viscosity 

 (rf), and that the relation between thickness [t), density (p) t 



and velocity of drawing v is t 2 = — — . 



If the solid is a fine wire of radius r, surface tension is 

 dominant and gravity negligible. If 7 is the surface tension, 



t f r)v\ 



the relation must be of the form - =/ — ). It is found 



r J \ 7 / 



(t)v \ nv 



— ) is of the form 4*8 — in c.g.s. units. 



These results apply to suspensions if (1) the diameter of 

 the suspended particles is not greater than t, (2) the effect 

 of the particles in increasing 77 is taken into account. 



There is no evidence of any special cohesion between 

 solids and liquids wetted by them other than that which 

 prevents slipping at the interface. 



In many important industrial processes solid surfaces 

 are coated with a layer of liquid by drawing them out of a 

 bath of the liquid. The enamelling of wires or tubes is such 

 a process, and so in all essentials is painting with a brush. 

 We are aware of no theory or even complete experimental 

 investigation directed to determine how the thickness of the 

 liquid layer produced in such circumstances varies with the 

 properties of the liquid, the solid surface, the velocity of 

 drawing, and other possible factors. The experiments 

 described in this paper show that the matter is surprisingly 

 simple. 



* Communicated by the Director. 



