Evaporative Flux Reversal of Binary Droplets: From Edge to Apex

In this Letter, we directly measure the spatial variation of the saturation concentration along the interface of a binary droplet, showing that it is not uniform. This observation challenges the core assumption of classical evaporation theory, where a uniform interfacial saturation concentration leads to the edge-dominant evaporative flux responsible for the well-known coffee-ring effect in single-component droplets. Our interferometric measurements reveal that this assumption breaks down in multicomponent droplets, requiring a different framework to describe their evaporation behavior. By comparing the local evaporative flux with internal flow fields obtained using micro-particle image velocimetry, we show that changes in interfacial composition and flow pattern are closely coupled, leading the maximum evaporative flux to shift away from the contact line during evaporation. Recognizing that the maximum flux no longer resides at the edge is therefore essential for developing a correct and predictive understanding of evaporation in binary and more complex multicomponent droplets.