<p>Particle deposition in microfluidic channels can lead to clogging and performance degradation. Despite extensive studies on flow and particle transport, it is still unclear how changes in flow features caused by viscosity and channel geometry influence deposition behavior. In this study, we investigate particle deposition in expansion–contraction microchannels by varying fluid viscosity and cavity length using microfluidic experiments and numerical simulations. With the cavity length fixed at 150&#xa0;μm, the viscosity of the glycerin solution is increased by increasing the glycerin concentration from 40 to 70 wt%. As viscosity increases, particle deposition decreases in the straight regions (both upstream and downstream), whereas it increases in the cavity region. To interpret these trends, two-dimensional steady-state flow simulations are conducted to analyze velocity field, wall shear stress, recirculation region, and stagnant flow region. Furthermore, at fixed fluid viscosity, the cavity length is varied from 50 to 200&#xa0;μm, along with a contraction channel corresponding to an infinite cavity length. Varying cavity length produced a non-monotonic deposition trend with a minimum at 150&#xa0;μm, which is associated with geometry-dependent changes in local flow structure, such as recirculation region. These findings suggest that viscosity and channel geometry influence particle deposition and provide practical guidance for controlling deposition in microchannel-based processes.</p> Graphical abstract <p></p>