Self-Organization of Soft Hydrogel Microspheres during the Evaporation of Aqueous Droplets

The
unique drying behavior of aqueous droplets that contain soft
hydrogel microspheres (microgels) upon evaporation was systematically
investigated. Compared to the ring-shaped deposits that are obtained
from drying solid microsphere dispersions, we have previously reported
that uniformly ordered thin films are obtained from drying ∼1.2
μm-sized poly­(N-isopropyl acrylamide) microgel
dispersions. In the present study, we thoroughly investigated several
hitherto unexplored aspects of this self-organization, such as the
effect of the size, chemical structure, and “softness”
of the microgels (or rigid microspheres). For the macro- and microscopic
observation of the drying behavior of various microsphere dispersions,
an optical microscope and a digital camera were employed. The results
suggested that the convection in the aqueous droplets plays an important
role for the transportation of the microgels to the air/water interface,
where the softness and surface activity of the microgels strongly
affects the adsorption of the microgels. On the basis of these discoveries,
a design concept for the rapid formation of uniform thin films of
soft microgels was proposed.