Using a special double-layer membrane to avoid interaction among flow fields generated by different pores, we have, for the first time, observed the predicted discontinuous first-order transition in ultrafiltration of flexible linear polymer chains. Namely, the chain could pass through a pore much smaller than its unperturbed radius only when the flow rate is higher than a certain value. When only one chain and one pore are considered in theory, such a threshold is surprisingly independent of both the chain length and the pore size. Our results reveal that for a membrane with many pores and at a microscopic flow rate ($q$) lower than the threshold, the inevitable blocking of some pores by longer nonstretched coiled chains increases $q$ in those unblocked pores because the macroscopic flow rate ($Q$) is a constant. Long chains have two populations, coiled and stretched, in a real ultrafiltration experiment when $q$ is lower than the threshold.

• Received 1 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.237801