Rideal-Washburn equation

For horizontal capillaries (gravity neglected), the depth of penetration 1 in time t is given by the Rideal-Washburn equation [15,16], 

To enhance the rate of penetration, y y has to be made as high as possible, and both 9 and p as low as possible. 

For dispersion of powders into liquids one should use surfactants that lower 9 while not reducing y y too much. The viscosity of the liquid should also be kept at a minimum. Thickening agents (such as polymers) should not be added during the dispersion process. It is also necessary to avoid foam formation during the dispersion. 

For a packed bed of particles, r may be replaced by K, which contains the effective radius of the bed and a turtuosity factor, which takes into account the complex path formed by the channels between the particles, i.e.. 

Thus a plot of versus t gives a straight line, from the slope of which one can obtain 6. 

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The rate of penetration of the liquid in a powder aggregate or agglomerate can be applied to measure the contact angle of the liquid on the powder surface. For horizontal capillaries (gravity neglected), the depth of penetration I in time t is given by the Rideal-Washburn equation [15, 16],... 

Thus, a plot of P versus t will give a straight line, from the slope of which 9 can be obtained. The Rideal-Washburn equation can be apphed to obtain the contact angle of liquids (and surfactant solutions) in powder beds however, k should first be obtained by using a liquid that produces a zero contact angle. This is discussed below. 

Penetration appears to take place by a capillary rise mechanism in such a case, the height of rise h of a fluid of viscosity rj ascending a capillary pore of radius r and contact angle 6 is given by a relationship such as the Rideal-Washburn equation (Rideal, 1922 Washburn, 1921) ... 

The Rideal Washburn equation can be applied to obtain the contact angle of liquids (and surfactant solutions) in powder beds, k should first be obtained using a... 

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