  Following Gibb's interpretation of an interface as a dividing surface, we
derive a model for the microscopic dynamic contact angle by writing a force
balance for a control volume encompassing the interfaces and the contact line.
In doing so we identify that, in addition to the surface tension of respective
interfaces, the gradient of surface tension plays an important role in
determining the dynamic contact angle. This is because not only does it
contribute towards an additional force, but it also accounts for the deviation
of local surface tension from its static equilibrium value. It is shown that
this gradient in surface tension can be attributed to the convective
acceleration in the vicinity of the contact line, which in turn is a direct
result of varying degree of slip in that region. In addition, we provide
evidence that this gradient in surface tension is one of the key factors
responsible for the difference in contact angle at the leading and trailing
edge, of a steadily moving contact line. These findings are validated using
molecular dynamics simulations.
