A novel microfluidic device that subjects a solution to a constant shear flow was developed. By taking advantage of the linear velocity profile in a lid driven flow configuration, small volumes (10-5 L) can be subjected to a constant shear profile with a shear rate between 0.1 and 100 s-1 at accurately controlled temperatures between 20 and 50 °C. The tunable shear can be maintained for extensive and fully controlled times. A dedicated microscope setup for visualization enables the on-chip detection of micron-sized crystals, particles, and aggregates. The influence of shear on the crystallization process of the reference protein lysozyme was studied. The results indicate that shear rates between 1 and 10 s-1 decrease solubility and promote nucleation not only in the supersaturated and metastable zones of the phase diagram, but also in the undersaturated zone. A monotonically increasing nucleation rate was observed for shear rates between 1 and 10 s-1. It is anticipated that the presented methodology can shed light on a variety of phase transitions that are influenced by flow.