[Tex/LaTex] Problem of different spaces between subscripts and letters in math mode

kerningmath-modespacingsubscripts

In math mode, the spaces between subscripts and letters are not always the same, but depending on the preceding letters!

See e.g. (different colours indicate different spaces)enter image description here

Is it the feature of packages {amssymb} {amsmath} or ….

Equations using {newtxmath} math font:

    \begin{equation}
    \left.
    \begin{array} {c}
    \displaystyle \frac{\partial\epsilon_f}{\partial t} + \nabla \cdot (\epsilon_f \boldsymbol{u}_f) = 0 \\
    \displaystyle \frac{\partial  (\rho_f \epsilon_f \boldsymbol{u}_f)}{\partial t} + \nabla \cdot (\rho_f \epsilon_f \boldsymbol{u}_f \boldsymbol{u}_f) = -\epsilon_f \nabla p + \mu \nabla^2(\epsilon_f \boldsymbol{u}_f) + \boldsymbol{f}_b
    \end{array}
    \right\}
    \end{equation}

Is there any global setting/solution for this issue, i.e. to adjust the spacing automatically depending on the letters and subscripts?

EDIT: put {bm} package after amsmath and font packages will significantly improve the spacing.

Best Answer

After some testing I found that loading \usepackage{bm} significantly improves the spacing. As David points out in the comment, the kerning is possible, but grouping plays a role here. Obviously his package bm does better job on \boldsymbol than amsmath. Just remember to load bm after amsmath and after your font packages!