Deepak K. Gupta, D. Karthickeyan, B. V. R. Tata, T. R. Ravindran
Poly(N-isopropylacrylamide) (PNIPAM)-based microgels respond to temperature and exhibits a transition from swollen to deswollen state upon variation of temperature, which is known as volume phase transition (VPT). Dynamic light scattering (DLS) is a popular technique to identify the volume phase transition temperature (VPTT) of microgel particles, which measures variation of particle size with temperature in a suspension having microgel particle concentration of 107–108 particles/cm3. Here, we employ optical tweezers to trap a single microgel particle and identify its VPTT by measuring the lateral trap stiffness, κ as a function temperature. It is shown that near the VPTT, κ increases gradually upon increasing temperature, which is due to a gradual decrease in particle size with simultaneous increase in its refractive index.