= Solution
\b[Positive <dielectrophoresis> acts up the gradient of the squared RMS field, towards stronger-field regions; negative <dielectrophoresis> acts down that gradient, towards weaker-field regions.] The sign is the sign of $\Re\widetilde K(\omega)$.
The force is not generally tangent to an <electric field> line. Field lines show the direction of the field itself, whereas $\nabla |\mathbf E_{\rm RMS}|^2$ describes how its strength changes in space. A spatially uniform field gives zero dipole-order dielectrophoretic force even when it is large. Reversing the field each half-cycle does not reverse this time-averaged force, because the relevant intensity-like quantity is quadratic.
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