Multi-stimuli responsive nanofluid with easy-to-visualize structural color patterns

Abstract

We present the details of a multi- stimuli responsive nanofluid with easy-to-visualize structural color patterns. We have used a thermo-responsive polymer, Poly (N-isopropylacrylamide)(PNIPAM) and superparamagnetic nanoparticle to realize both thermal and magnetic actuation of the nanofluid. PNIPAM molecules adsorbed at the oil-water (O/W) interface acts as a stabilizer cum thermo-responsible moiety, while the superparamagnetic nanoparticles make the nanofluid magnetically responsive. Under a constant magnetic field and in the temperature range of 5–30°C, the interdroplet separation is reversibly tuned from 82 to 52nm. The reversible swelling and collapse of polymer adsorbed at the O/W interface is manifested from the Bragg peak shift at a constant magnetic force. Above the lower critical solution temperature (>35°C), a strong attraction between adsorbed PNIPAM layers caused the bridging of PNIPAM covered droplets, which was evident from the disappearance of Bragg peak after the removal of magnetic stimulus and the presence of bridged long flexible chains of drops. It is observed that the incorporation of a small amount of anionic surfactant monomers dramatically increases the volume phase transition, the repulsive interaction forces and the interparticle tunability due to strong association of PNIPAM and sodium dodecyl suphate (SDS). Our results suggest that by using PNIPAM and an anionic surfactant, the interparticle interactions and optical properties of magnetically polarizable nanofluids can be reversibly tuned. Thermo-magneto responsive nanoemulsions offer exciting applications in optical devices (2D and 3D photonic crystals, optical gratings, body temperature sensor etc.), smart drug delivery systems and sensors. Besides, they are wonderful model system to probe temperature induced conformational changes under different physiological conditions.