Here we show that
microgels trapped at a solid wall can issue liquid
flow and transport over distances several times larger than the particle
size. The microgel consists of cross-linked poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-AA)
polymer chains loaded with cationic azobenzene-containing surfactant,
which can assume either a trans- or a cis-state depending on the wavelength of the applied irradiation. The
microgel, being a selective absorber of trans-isomers,
responds by changing its volume under irradiation with light of appropriate
wavelength at which the cis-isomers of the surfactant
molecules diffuse out of the particle interior. Together with the
change in particle size, the expelled cis-isomers
form an excess of the concentration and subsequent gradient in osmotic
pressure generating a halo of local light-driven diffusioosmotic (l-LDDO) flow. The direction and the strength of the l-LDDO depends on the intensity and irradiation wavelength,
as well as on the amount of surfactant absorbed by the microgel. The
flow pattern around a microgel is directed radially outward and can
be maintained quasi-indefinitely under exposure to blue light when
the trans-/cis-ratio is 2/1, establishing a photostationary
state. Irradiation with UV light, on the other hand, generates a radially
transient flow pattern, which inverts from inward to outward over
time at low intensities. By measuring the displacement of tracer particles
around neutral microgels during a temperature-induced collapse, we
can exclude that a change in particle shape itself causes the flow,
i.e., just by expulsion or uptake of water. Ultimately, it is its
ability to selectively absorb two isomers of photosensitive surfactant
under different irradiation conditions that leads to an effective
pumping caused by a self-induced diffusioosmotic flow.