A microfluidic device includes: a first microfluidic channel; a second microfluidic channel extending along the first microfluidic channel; and a first array of islands separating the first microfluidic channel from the second microfluidic channel, in which each island is separated from an adjacent island in the array by an opening that fluidly couples the first microfluidic channel to the second microfluidic channel, in which the first microfluidic channel, the second microfluidic channel, and the islands are arranged so that a fluidic resistance of the first microfluidic channel increases relative to a fluidic resistance of the second microfluidic channel along a longitudinal direction of the first microfluidic channel such that, during use of the microfluidic device, a portion of a fluid sample flowing through the first microfluidic channel passes through one or more of the openings between adjacent islands into the second microfluidic channel.