A heat exchanger comprises at least one first fluid passage contiguous with at least one second fluid passage. The first fluid passage is separated from the second fluid passage by a common boundary, the boundary having a first surface and a second surface. The first surface is described by a plurality of curvilinear surfaces joined at the ends thereof into a sinuous curvilinear surface and the second surface is described by a plurality of curvilinear surfaces joined at the ends thereof into a sinuous curvilinear surface. The curvilinear surfaces of the first and the second curvilinear surfaces are defined by a portion of an inverse square curve.