A microfluidic assay device comprising: an inlet in fluid connection with a first portion of a channel, the inlet configured to receive a liquid; a second portion of the channel connected to the first portion at a junction; and magnetically susceptible particles disposed in the first portion of the channel, characterized in that the first portion of the channel has a main channel height h 1 and a channel height h' 1 at the junction, wherein h 1 >h' 1 and wherein at the junction, the first portion of the channel has height h' 1 and the second portion of the channel has height h 2, wherein h 2 >h 1 >h' 1 and the ratio h' 1:h 2 is at least 1:2 wherein the device is configured to form, with a liquid received by the inlet, a liquid interface proximal the junction, the device further characterized by: a reservoir containing a quantity of second liquid and configured to deliver second liquid released from the reservoir into the second portion of the channel such that the second liquid flows towards the junction, a single overflow channel comprising a reagent control zone having one or more detection electrodes to detect the reagent control signal which reagent control zone is located in the overflow channel at least about 20 mm downstream of the junction such that detection of the reagent control signal can take place in the second liquid that is free or substantially completely free of reagent, and at least one sensor configured on the second channel portion to detect a signal from the second liquid. an inlet in fluid connection with a first portion of a channel, the inlet configured to receive a liquid; a second portion of the channel connected to the first portion at a junction; and magnetically susceptible particles disposed in the first portion of the channel, characterized in that the first portion of the channel has a main channel height h 1 and a channel height h' 1 at the junction, wherein h 1 >h' 1 and wherein at the junction, the first portion of the channel has height h' 1 and the second portion of the channel has height h 2, wherein h 2 >h 1 >h' 1 and the ratio h' 1:h 2 is at least 1:2 wherein the device is configured to form, with a liquid received by the inlet, a liquid interface proximal the junction, the device further characterized by: a reservoir containing a quantity of second liquid and configured to deliver second liquid released from the reservoir into the second portion of the channel such that the second liquid flows towards the junction, a single overflow channel comprising a reagent control zone having one or more detection electrodes to detect the reagent control signal which reagent control zone is located in the overflow channel at least about 20 mm downstream of the junction such that detection of the reagent control signal can take place in the second liquid that is free or substantially completely free of reagent, and at least one sensor configured on the second channel portion to detect a signal from the second liquid. a reservoir containing a quantity of second liquid and configured to deliver second liquid released from the reservoir into the second portion of the channel such that the second liquid flows towards the junction, a single overflow channel comprising a reagent control zone having one or more detection electrodes to detect the reagent control signal which reagent control zone is located in the overflow channel at least about 20 mm downstream of the junction such that detection of the reagent control signal can take place in the second liquid that is free or substantially completely free of reagent, and at least one sensor configured on the second channel portion to detect a signal from the second liquid.