An arrangement in a capillary driven fluidic system for mixing a first fluid with a second fluid at a predetermined volume mixing ratio, the arrangement comprising: a mixing chamber including a main chamber and one or more inner chambers (130a, 130b), said main chamber and each of the one or more inner (130a, 130b) chambers being separated by a respective structure (124a,124b) each including at least one opening (126a, 126b) which allows for fluid communication between the main and the one or more inner (130a, 130b) chambers and which, during use, is arranged to generate a capillary pressure in the at least one opening (126a,126b) which is larger than a capillary pressure in the main chamber, wherein the mixing chamber extends in a longitudinal direction and the main chamber extends in said longitudinal direction along a full length of the mixing chamber, and wherein the mixing chamber is arranged to receive a first fluid so as to fill the main chamber and the one or more inner chambers (130a,130b), via the respective at least one opening (126a, 126b), with the first fluid, a capillary pump arranged to draw fluid from the main chamber after the main chamber and the one or more inner chambers of the mixing chamber have been filled with the first fluid, wherein the capillary pump is arranged to operate at a capillary pressure which is between the capillary pressure of the main chamber and the capillary pressure in the at least one opening (126a,126b) of each respective structure (124a, 124b) such that the main chamber but not the one or more inner chambers (130a,130b) is emptied of the first fluid, and wherein the mixing chamber is arranged to receive a second fluid so as to fill the main chamber with the second fluid after the main chamber has been emptied of the first fluid, such that the first fluid in the one or more inner chambers (130a,130b) and the second fluid in the main chamber are enabled to mix through the at least one opening (126a, 126b) of the respective structure (124a,124b). a mixing chamber including a main chamber and one or more inner chambers (130a, 130b), said main chamber and each of the one or more inner (130a, 130b) chambers being separated by a respective structure (124a,124b) each including at least one opening (126a, 126b) which allows for fluid communication between the main and the one or more inner (130a, 130b) chambers and which, during use, is arranged to generate a capillary pressure in the at least one opening (126a,126b) which is larger than a capillary pressure in the main chamber, wherein the mixing chamber extends in a longitudinal direction and the main chamber extends in said longitudinal direction along a full length of the mixing chamber, and wherein the mixing chamber is arranged to receive a first fluid so as to fill the main chamber and the one or more inner chambers (130a,130b), via the respective at least one opening (126a, 126b), with the first fluid, a capillary pump arranged to draw fluid from the main chamber after the main chamber and the one or more inner chambers of the mixing chamber have been filled with the first fluid, wherein the capillary pump is arranged to operate at a capillary pressure which is between the capillary pressure of the main chamber and the capillary pressure in the at least one opening (126a,126b) of each respective structure (124a, 124b) such that the main chamber but not the one or more inner chambers (130a,130b) is emptied of the first fluid, and wherein the mixing chamber is arranged to receive a second fluid so as to fill the main chamber with the second fluid after the main chamber has been emptied of the first fluid, such that the first fluid in the one or more inner chambers (130a,130b) and the second fluid in the main chamber are enabled to mix through the at least one opening (126a, 126b) of the respective structure (124a,124b).