  Using the Helmholtz decomposition of the vector field of folding fluxes in a
two-dimensional space of collective variables, a potential of the driving force
for protein folding is introduced. The potential has two components. One
component is responsible for the source and sink of the folding flows, which
represent, respectively, the unfolded states and the native state of the
protein, and the other, which accounts for the flow vorticity inherently
generated at the periphery of the flow field, is responsible for the
canalization of the flow between the source and sink. The theoretical
consideration is illustrated by calculations for a model $\beta$-hairpin
protein.
