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\begin{itemize}  \item I think $t_{\rm h}\sim \ell/c_{\rm s}$ on physical grounds, but Payam says $t_{\rm h}\sim \ell^2\rho/\eta$ from dimensional analysis. Are these compatible? \begin{itemize}  \item Sound speed shouldn't depend on viscosity at all unless we are near supersonic flow.  \item \url[Or \href{http://iopscience.iop.org/0370-1301/66/5/303/pdf/0370-1301_66_5_303.pdf}{Or  does it?]{http://iopscience.iop.org/0370-1301/66/5/303/pdf/0370-1301_66_5_303.pdf} it?}  \end{itemize}.  \item Payam has (equation 12) $v_{\rm DNA}=F_{\rm ext} \ln(\ell/d)/\eta\ell - \lambda E\ln(1+r_{\rm D}/d)/\eta$, which we use to calculate $t_{\rm D}$. He finds $t_{\rm D}\sim \ell\eta/E\lambda$ -- whence this simplification?