and is the "time constant" of the rotation, or the time taken for the initial angle 0O to be reduced to 1/e of its value.
If the alignment process is to be reasonably complete, the particle must fall through water for at least time t0. From Stokes' law, Stacey (1963) has shown that the time of fall t of a spherical grain through a depth h of water is given by
d (P-Po)g where (p-p0) is the density difference between the grain and the water and g is the acceleration due to gravity. There is thus some critical height h0 through which the grain must fall for the time t0 to have elapsed. Equating (2.3.34) and (2.3.35), h0 is given by
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