Generation of Local Diffusioosmotic Flow by Light Responsive Microgels

Abstract

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.

Making microgels photo-responsive by complexation with a spiropyran surfactant

“…Microgels are prepared utilizing free-radical polymerization in a droplet-based microfluidic system by varying the monomer (NIPAM) and co-monomer (AAc) concentrations as described elsewhere. 46 In short, a mixture of NIPAM, AAc, BIS and APS dissolved in water is degassed under Ar and transferred into glass syringes, that is used as dispersed phase. At the cross-junction of the PDMS device, where fluorocarbon oil is used as the continuous phase, water-in-oil droplets are formed by channeling the flow of the water phase onto the oil phase to generate emulsions.…”

“…39–42 In the case of an azobenzene containing surfactant, the more stable trans -state prefers to be within the microgel due to energy gain from micellization in the gel interior, 43 while the cis -state diffuses out of the particle. 44–46…”

“…[39][40][41][42] In the case of an azobenzene containing surfactant, the more stable trans-state prefers to be within the microgel due to energy gain from micellization in the gel interior, 43 while the cis-state diffuses out of the particle. [44][45][46] In this paper, we report on the triggering of microgel photoresponsivity by coupling it with a spiropyran (SP) containing surfactant, where the SP group is incorporated at the end of the hydrocarbon tail of the cationic surfactant. 47 The spiropyran/ merocyanine-containing (SP/MC) amphiphiles have been utilized for many applications such as micelle formation/decomposition during photo-isomerization, [48][49][50] achieving control over shape and color-changing of block copolymer particles, 51 manipulation of bubble-propelled micromotors, 52 construction of a photo-responsive membrane for tailored drug delivery, 53 and synthesis of photochromic polymersomes for reversible permeability, 54 as well as synthesis of photo-initiators for polymerization.…”

Versatile Microfluidics Separation of Colloids by Combining External Flow with Light‐Induced Chemical Activity

“…Under exposure to UV light, the flow is transient and decays when the particles empty their load of cis-isomers. [38] Such local DO flow can, thus, be generated at any surface absorbing trans-and repelling cis-isomers; [39] examples here are porous particles, [35] microgels, [40] hair, nails, wood sticks, sand, soil, etc. [41] In this work, we show that by combining the chemical activitycontrolled l-LDDO with a pressure-driven flow within a channel, it is possible to achieve the separation of micron-sized particles sedimented at the channel wall.…”

Thermal/Near‐Infrared Light Dual‐Responsive Reconfigurable and Recyclable Polythiourethane/CNT Composite with Simultaneously Enhanced Strength and Toughness

“…However, the solid-state plasticity of the thermoset polymers previously reported were mostly thermal-responsive, which is too dependent on external conditions and does not allow for precise local shape modulation. To realize the non-contact, [16][17][18] remotely controllable, [19] and locally precise shape manipulation, [20] the light-responsive solid-state plasticity of the thermoset polymers should be investigated. Currently, the light-responsive properties of the materials can be mainly obtained by incorporating photothermal fillers such as MXene, [21][22][23][24][25] graphene, [26][27][28][29] carbon nanotubes, [30][31][32][33][34] gold nanoparticles [35,36] into the polymers.…”