Synthetic self-propelled particles that migrate upstream mimic bacteria.

that was initially suspended. This highly nonuniform deposition is known as the coffee-ring effect; it is produced when the drop edges are pinned during evaporation and subsequent radial capillary fl ows from the drop center carry suspended solutes to the drop perimeter. [5][6][7] The coffeering effect, however, can be ameliorated in aqueous systems of large (>10 µm) suspended spheres, [ 8 ] in suspensions of ellipsoidal particles with strong capillary interactions, [ 9 ] via electrowetting, [ 10 ] in systems with strong attractive forces between particles and substrate, [ 11 ] and in the presence of surfactants that lead to strong Marangoni fl ows, [ 12,13 ] among other techniques. [14][15][16] In these cases, the drying drop system provides experimenters with an opportunity to make uniform thin particle coatings and fi lms.The present contribution builds on investigations of Horigome and Suzuki, which demonstrated that drying aqueous suspensions of PNIPAM particles on polystyrene substrates did not yield typical coffee-ring deposition. [ 17 ] Rather, the hydrogel particles tend to act as amphiphilic moieties, populating the air-water interface fairly uniformly during evaporation. [ 18 ] This behavior, also observed for linear PNIPAM polymers, [ 19,20 ] induces a predominantly uniform deposition of hydrogel particles on the substrate. In our work, we employ micrometersize (diameter d = 1-4 µm) PNIPAM microgel particles with comonomer 2-aminoethylmethacrylate hydrochloride (AEMA) in the suspension. Importantly, the AEMA comonomer introduces primary amine groups onto the microgel particle surfaces that can be used for subsequent cross-linking. Much smaller (<100 nm) PNIPAM particles are feasible, [ 21 ] but systematic studies on the introduction of amine groups still have to be performed. On the other end of the spectrum, we can prepare functionalized particles with amine groups up to approximately 10 µm with the referenced methods. Even larger particles are achievable using microfl uidic techniques.After the water has evaporated, a mostly uniform microgel particle fi lm remains on the substrate. It is kept untouched for another 24 h to ensure cross-linking of neighboring particles by glutaraldehyde that is also mixed into the original droplet suspension. The resultant hydrogel-particle fi lm is temperature-sensitive because the hydrogel particles that compose it are temperature-sensitive. Thus, fi lm size, shape, porosity, and even optical properties are readily varied with temperature control.Our novel responsive fi lms can be utilized as membranes that differ from fi lm structures that have been previously A simple method to prepare temperature-sensitive fi lms composed of micrometer-sized colloidal hydrogel particles using evaporating drops of colloidal suspensions is demonstrated. The fi lms range in thickness from a monolayer to approximately fi fty particle diameters depending on initial particle volume fraction. Sessile droplets of hydrogel-particle suspensions are evaporated on silicon wafers. The fi lm i...

Most drying colloidal drops produce a useless stain. In article 1500371, T. Still and co‐workers show how a simple one‐step drying process can be utilized to prepare colloidal hydrogel films that consist of cross‐linked micron‐size poly(N‐isopropyl acrylamide) (PNIPAM) particles. These colloidal hydrogel particles have the unique ability to change their size with temperature. Thus, the PNIPAM films produced by drying drops reversibly change their size as well as their mechanical and optical properties as a function of temperature. Ultimately, these films present new materials with adjustable porosity, size, strength and light transmission.

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