= Solution
First hold the surface <electric charge> fixed and increase the bulk salt concentration. Since $\kappa^2\propto c$, the <Debye–Hückel screening length> decreases as $c^{-1/2}$. For surface gap $h=R-2a\geq0$,
$$
U_{\mathrm{el}}(R)\propto\frac{e^{-\kappa h}}{(1+\kappa a)^2},
\qquad
\frac{\partial\log U_{\mathrm{el}}}{\partial\kappa}
=-h-\frac{2a}{1+\kappa a}<0.
$$
Thus both the range and the strength of repulsion decrease. In the repulsive <phase diagram>, the sample moves downwards at fixed <volume fraction> and can melt from a <crystal> into a fluid. Because screening length is already a diagram axis, changing salt moves the state through that diagram rather than independently changing its fixed-charge coexistence curves. If <Van der Waals forces> become dominant, however, adding salt may instead destabilize the dispersion and cause aggregation. These are distinct physical regimes.
Second increase $\sigma$ at fixed salt concentration and particle size. The <electric charge> is $Q=4\pi a^2\sigma$, so the pair repulsion increases approximately as $\sigma^2$ within the <Debye–Hückel approximation>, while the reservoir screening length remains unchanged. Freezing then becomes possible at lower <volume fraction> or shorter screening length. At fixed $\lambda_D$, the coexistence curves shift towards lower $\eta$; a previously fluid sample may crystallize.
These statements assume fixed, uniform charge and a salt reservoir dominating the screening. Strong charging requires nonlinear screening and possibly charge regulation. At very low added salt or appreciable particle concentration, particle-released <counterions> also contribute to <ionic strength>, so the screening length is no longer independently fixed by the added salt alone.
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