(a) The objects (for example, gold nanoparticles or liposomes) are linked to a fluid interface (for example, a fluid-phase SLB) within a microfluidic channel (using a design as recently described27). The linking confines the object’s movement into two dimensions, but maintains its ability to move freely. Its movement is monitored from below, for example using scattering, confocal or TIRF imaging. This is demonstrated in b for the particular example of streptavidin-functionalized gold nanoparticles (hydrodynamic radius 30 nm) that are linked to biotinylated lipids in the SLB and label-free monitored using surface-enhanced ellipsometric contrast (SEEC) imaging. Application of a flow through the channel creates a shear force, which depends on the flow rate (applied to the channel; 5 μl min−1, 10 μl min−1 and 15 μl min−1 from top to bottom in b) and the object’s hydrodynamic size. Shown are only the first 100 frames (=steps) of each trajectory corresponding to an observation time of 3 s. The red trajectory is further analysed in Fig. 2.