Deformation and rotation process of the boundary or interface between two fluids with the same properties was studied in a three-dimensionally bent channel that consisted of two straight channels with square cross-section interconnected via a circular channel. A planar fluid interface was formed in the upstream straight channel by feeding two fluids at the same rate. The fluid interface thus formed was visualised in the upstream and downstream channel cross-sections by using LIF method. In the fully developed flow at Re < 2, the cross-sectional interfacial shape in the downstream channel was slightly curved and inclined. By applying linear approximation to the curved fluid interface, it was found that the corkscrew rule was applicable to the rotation of fluid interface: the mid-line interface in the upstream cross-section rotates through the following 3D-bent channels by the crossed axis angle between upstream and downstream channels. Based on this rule, a fundamental concept is presented for designing split-and-recombine mixers that performs 180 rotation of fluid interface, and possible channel configurations composed of 3D bent channels are proposed. By experimentally testing an S&R mixer combined with two 3D channels of 120 and 60 bending angles, it has been confirmed that the design concept is realisable. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.