Example embodiments relate to fast sample loading microfluidic reactors and systems. One embodiment includes a microfluidic device. The microfluidic device includes a reaction chamber allowing reacting of at least one fluid material. The microfluidic device also includes at least two fluidic channels coupled to the reaction chamber for providing a fluid to and exiting a fluid from, respectively, the reaction chamber. Each fluidic channel includes an inlet and an outlet. Each fluidic channel is configured such that when a first fluid is provided in the reaction chamber via that fluidic channel, the first fluid exits the reaction chamber via the outlet of at least one other fluidic channel when the reaction chamber is filled, thereby preventing a second fluid from the at least one other fluidic channel, when present in the inlet, from diffusing into the reaction chamber.