A microfluidic device includes: a microchannel defining a flow path; a Brownian motor structure comprising two or more sorting channels having distinct ratchet topographies, the Brownian motor structure in fluid communication with the microchannel; and a filter extending transversely to the microchannel, the filter configured to filter particles, subject to sizes thereof, in a liquid advancing along the flow path, whereby smaller particles of the liquid can pass downstream of the filter in the flow path, and larger particles of the liquid are directed to the Brownian motor structure to be sorted out according to sizes thereof via the sorting channels.