0
$\begingroup$

Given the Frank-De Gennes free energy $F = \int f(\boldsymbol{p},\nabla\boldsymbol{p}, ...)\ d\boldsymbol{x},$ for liquid crystals (see De Gennes-Prost, p. 107, formula 3.21), the vector

$$h_{i}=-\frac{\delta F}{\delta P_{i}}=-\frac{\partial f}{\partial P_{i}}+\partial_{j}\frac{\partial f}{\partial\partial_{j}P_{i}}$$

is usually called molecular field, a "notation derived from magnetism". What is the physical meaning of this vector in this context? Does it have any sense to call it molecular also here?

$\endgroup$
  • $\begingroup$ "Molecular field is an internal mean effective field resulting from the interaction of dynamic variables in the a system" --Chaikin&Lubensky. I suppose it is the interaction between molecules (or lattice sites) that gives meaning to the "molecular" part. As an example, in the context of the Ising model, this would be $h + zJm$, where $h$ is the external field, $z$ the # of neighbors, $J$ the interaction parameter and $m$ the mean field value of the order parameter. $\endgroup$ – alarge Jul 27 '14 at 7:45
0
$\begingroup$

I think the comment from @alarge is correct. Thus, this is the "field" that a molecules feels and tends to align itself.

$\endgroup$
  • $\begingroup$ This is not an answer. It is a comment. $\endgroup$ – Bill N Sep 30 '15 at 13:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.