In this talk, I will cover two problems, both about the structure a small obstacle leaves in its wake. In the first problem, hydrogel posts photopatterned in a microchannel release solute into a flow. A fluorescent solute lets us image the concentration boundary layer directly; an invisible solute leaves its record in the suspended colloids, which diffusiophoresis sweeps out of the plume to leave a particle-free “shadow” whose width reads out the mass transfer from the post, a hot-wire anemometer for chemical exchange. Reversing the flow around an asymmetric post then tests a 1967 theorem of Brenner’s on which intuition and theory disagree. In the second problem, a planar ice front engulfing a microfabricated post exposes flat crystallographic facets that persist in its wake and hold unfrozen water after the front has moved on. The facets lie in the basal plane of the ice crystal, their length is set by the temperature field around the post, and pairs of posts can be designed so that merging facets seal off and compress a pocket of unfrozen water. In both problems, the wake makes visible what we could not otherwise see: the exchange of an invisible solute in one case, the crystallography and growth kinetics of a transparent ice crystal in the other.