  With the aim to parallelize and monitor biological or biochemical phenomena,
trapping and immobilization of objects such as particles, droplets or cells in
microfluidic devices has been an intense area of research and engineering so
far. Either being passive or active, these microfluidic devices are usually
composed of arrays of elementary traps with various levels of sophistication.
For a given array, it is important to have an efficient and fast immobilization
of the highest number of objects, while optimizing the spatial homogeneity of
the trapping over the whole chip. For passive devices, this has been achieved
with two-layers structures, making the fabrication process more complex. In
this work, we designed small microfluidic traps by single-layer direct laser
writing into a photoresist, and we show that even in this simplest case, the
orientation of the main flow of particles with respect to the traps have a
drastic effect on the trapping efficiency and homogeneity. To better understand
this phenomenon, we have considered two different flow geometries: parallel and
oblique with respect to the traps array, and compared quantitatively the
immobilization of particles with various sizes and densities. Using image
analysis, we show that diagonal flows gives a spatial distribution of the trap
loading that is more homogeneous over the whole chip as compared to the
straight ones, and by performing FEM and trapping simulation, we propose a
qualitative explanation of this phenomenon.
