deletions | additions       diff --git a/Swimmers.tex b/Swimmers.tex  index d1a3381..518b66a 100644  --- a/Swimmers.tex  +++ b/Swimmers.tex 
...
\end{enumerate}   \item Then consider a single active swimmer in a concentration gradient. Get superposition of two behaviours?  \end{enumerate} {\bf ANSWERS}     \subsubsection{Diffusio-phoresis}     For {\it charged} interactions, the mobility $\mu\propto 1/c$. This leads to diffusio-phoretic velocity $V = D\del \log c$. This arises from the balance of osmotic and viscous forces in the Debye layer. The diffusion coefficient $D \sim T/\eta\ell_{\rm B}$, where $\ell_{\rm B}$ is the Bjerrum length (ratio of electrostatic to thermal energy).