A microfluidic device comprising: a first microfluidic channel; a second microfluidic channel arranged adjacent to the first microfluidic channel; a first array of more than two magnets arranged above the microfluidic channels (208a, 208b), wherein each magnet in the first array has a magnetic pole orientation that is opposite to a magnetic pole orientation of an adjacent magnet in the first array; and a second array of more than two magnets arranged beneath the microfluidic channels (208a, 208b), wherein each magnet in the second array has a magnetic pole orientation that is opposite to a magnetic pole orientation of an adjacent magnet in the second array, wherein the first array is aligned with respect to the second array such that magnetic fields emitted by the first array and the second array generate a magnetic flux gradient profile that extends through both the first_microfluidic channel and the second microfluidic channel, and wherein the magnetic flux gradient profile extends transverse to a central longitudinal axis of the microfluidic channels; characterized in that the absolute value of the magnetic flux gradient profile comprises at least two local minima, the two local minima are located within the microfluidic channels, and an interface between each magnet within the first array is aligned to a corresponding interface in the second array to produce two focus regions (216a, 216b), wherein a first focus region is a position within the first microfluidic channel that corresponds to a first local minimum in the magnetic flux gradient, and a second focus region is a position within the second microfluidic channel that corresponds to a second local minimum in the magnetic flux gradient. a first microfluidic channel; a second microfluidic channel arranged adjacent to the first microfluidic channel; a first array of more than two magnets arranged above the microfluidic channels (208a, 208b), wherein each magnet in the first array has a magnetic pole orientation that is opposite to a magnetic pole orientation of an adjacent magnet in the first array; and a second array of more than two magnets arranged beneath the microfluidic channels (208a, 208b), wherein each magnet in the second array has a magnetic pole orientation that is opposite to a magnetic pole orientation of an adjacent magnet in the second array, wherein the first array is aligned with respect to the second array such that magnetic fields emitted by the first array and the second array generate a magnetic flux gradient profile that extends through both the first_microfluidic channel and the second microfluidic channel, and wherein the magnetic flux gradient profile extends transverse to a central longitudinal axis of the microfluidic channels; characterized in that the absolute value of the magnetic flux gradient profile comprises at least two local minima, the two local minima are located within the microfluidic channels, and an interface between each magnet within the first array is aligned to a corresponding interface in the second array to produce two focus regions (216a, 216b), wherein a first focus region is a position within the first microfluidic channel that corresponds to a first local minimum in the magnetic flux gradient, and a second focus region is a position within the second microfluidic channel that corresponds to a second local minimum in the magnetic flux gradient.