function ws=stokes(d,s);
%function ws=stokes(d,[s]);
%  Calculate Stokes fall velocity
%    Input:   d = diameter (microns)
%             s = specific density (gm/cm3)  (2.65 by default)
%    Output:  fall velocity (microns/second)
if(nargin==1),
  s=2.65;
end
g=981.;  % gravity in CGS
nu=0.01;  % viscocity in CGS cm2/s
ws=1/18 * ((s-1)*981*(d/1e4).^2)/nu;  % cm/s
ws=ws*1e4;  % microns/s
