  The transport of bio-particles in viscous flows exhibits a rich variety of
dynamical behaviour, such as morphological transitions, complex orientation
dynamics or deformations. Characterising such complex behaviour under well
controlled flows is key to understanding the microscopic mechanical properties
of biological particles as well as the rheological properties of their
suspensions. While generating regions of simple shear flow in microfluidic
devices is relatively straightforward, generating straining flows in which the
strain rate is maintained constant for a sufficiently long time to observe the
objects' morphologic evolution is far from trivial. In this work, we propose an
innovative approach based on optimised design of microfluidic
converging-diverging channels coupled with a microscope-based tracking method
to characterise the dynamic behaviour of individual bio-particles under
homogeneous straining flow. The tracking algorithm, combining a motorised stage
and microscopy imaging system controlled by external signals, allows us to
follow individual bio-particles transported over long-distances with
high-quality images. We demonstrate experimentally the ability of the
numerically optimised microchannels to provide linear velocity streamwise
gradients along the centreline of the device, allowing for extended consecutive
regions of homogeneous elongation and compression. We selected three test cases
(DNA, actin filaments and protein aggregates) to highlight the ability of our
approach for investigating the dynamics of objects with a wide range of sizes,
characteristics and behaviours of relevance in the biological world.
