An aftertreatment system for a work vehicle includes: a fluid injector configured to inject a treatment fluid; a selective catalytic reducer (SCR) substrate; and a mixer fluidly coupled to the fluid injector and the SCR substrate and including at least one scroll defining a scroll open area and configured to receive a flow of exhaust gas and the treatment fluid. The mixer has a fixed wall and a plate displaceably coupled to the fixed wall, the plate being associated with the at least one scroll such that a change in a flow rate of exhaust gas into the at least one scroll passively causes displacement of the plate to adjust a size of the scroll open area.