Hot Brownian Motion of Thermoresponsive Microgels in Optical Tweezers Shows Discontinuous Volume Phase Transition and Bistability

Abstract

Microgels are soft microparticles that often exhibit thermoresponsiveness and feature a transformation at a critical temperature, referred to as the volume phase transition temperature. Whether this transformation occurs as a smooth or as a discontinuous one is still a matter of debate. This question can be addressed by studying individual microgels trapped in optical tweezers. For this aim, composite particles are obtained by decorating Poly‐N‐isopropylacrylamide (pNIPAM) microgels with iron oxide nanocubes. These composites become self‐heating when illuminated by the infrared trapping laser, performing hot Brownian motion within the trap. Above a certain laser power, a single decorated microgel features a volume phase transition that is discontinuous, while the usual continuous sigmoidal‐like dependence is recovered after averaging over different microgels. The collective sigmoidal behavior enables the application of a power‐to‐temperature calibration and provides the effective drag coefficient of the self‐heating microgels, thus establishing these composite particles as potential micro‐thermometers and micro‐heaters. Moreover, the self‐heating microgels also exhibit an unexpected and intriguing bistability behavior above the critical temperature, probably due to partial collapses of the microgel. These results set the stage for further studies and the development of applications based on the hot Brownian motion of soft particles.

Light-to-Heat Conversion of Optically Trapped Hot Brownian Particles

“…We interpret this as a reduction of the solvent viscosity due to the rise of the temperature of the trapped particle, which consequently heats up the surrounding liquid. Accepting that the temperature can be linearly related to the laser power in the trap P as T = T 0 + BP , , where T 0 is the chamber temperature before laser heating, the experimental data are nonlinearly fitted to the function P f c = C η ( T 0 + B P ) where B and C are fitting parameters. The shear viscosity was assume to follow a Vogel–Fulcher–Tammann (VFT) dependence on temperature, i.e, η ( T ) = η ∞ .25em exp [ A V F / false( T − T normalV normalF false) ] where the coefficients A VF , η ∞ and T VF are obained from ref .…”

“…Further applications of the protocol initiated in this work might yield a reinterpretation or validation of the trapping experiments on self-emissive nanothermometers. 59 On a final note, in light of the importance of the HBM theory to elucidate nanothermometric data coming from optical trapping setups, it seems urgent to generalize it to incorporate the local heating contribution of both the nearby particles and solvent and the effects of the particle softness 40 into the formalism.…”

“…We interpret this as a reduction of the solvent viscosity due to the rise of the temperature of the trapped particle, which consequently heats up the surrounding liquid. Accepting that the temperature can be linearly related to the laser power in the trap P as T = T 0 + BP, 23,40 where T 0 is the chamber temperature before laser heating, the experimental data are nonlinearly fitted to the function…”

Mean-field models for the chemical fueling of transient soft matter states

“…1, or, in general, any meaningful structural (order) parameter. In order to allow for state transitions of the system (as, e.g., in the hydrogel volume transition, 30,31 ) the coordinate is assumed to live in a bimodal energy landscape, H 0 ðQÞ ¼ AðDQÞ 2 þ BðDQÞ 4 , which we model by a simple quartic form as put forward in the simplest case by Landau to model phase transitions. 25 Here, DQ = Q À Q c , and A, B, and Q c describe the intrinsic energy landscape, with Q c being the center of the symmetric quartic form.…”