  We study numerically joint mixing of salt and colloids by a chaotic velocity
field $\mathbf{V}$, and how salt inhomogeneities accelerate or delay colloid
mixing by inducing a velocity drift $\mathbf{V}_{\rm dp}$ between colloids and
fluid particles as proposed in recent experiments \cite{Deseigne2013}. We
demonstrate that because the drift velocity is no longer divergence free, small
variations to the total velocity field drastically affect the evolution of
colloid variance $\sigma^2=\langle C^2 \rangle - \langle C \rangle^2$. A
consequence is that mixing strongly depends on the mutual coherence between
colloid and salt concentration fields, the short time evolution of scalar
variance being governed by a new variance production term $P=- \langle C^2
\nabla \cdot \mathbf{V}_{\rm dp} \rangle/2$ when scalar gradients are not
developed yet so that dissipation is weak. Depending on initial conditions,
mixing is then delayed or enhanced, and it is possible to find examples for
which the two regimes (fast mixing followed by slow mixing) are observed
consecutively when the variance source term reverses its sign. This is indeed
the case for localized patches modeled as gaussian concentration profiles.
