Provided herein are systems, devices, and methods for generating thermal gradients in channels and uses thereof. In particular, provided herein are system, methods, and devices employing first and second thermal layers positioned around a channel in order to create a thermal gradient across a cross-section of the channel having, for example, a nucleic acid denaturation zone, a nucleic acid annealing zone, and a nucleic acid polymerization zone. Such devices find use in, for example, nucleic acid amplification procedures, including digital polymerase chain reaction (dPCR) to temperature cycle droplets for amplification of nucleic acid templates within the droplets.