Optical Trapping of Thermo-responsive Microgel Particles by Holographic Optical Tweezers

Abstract

Holographic Optical Tweezers (HOT) is a technique in which the phase of trapping laser is modulated for generating steerable, multiple optical traps in a sample chamber. An indigenously developed HOT set‐up at Raja Ramanna Centre for Advanced Technology, Indore has been used to trap thermo‐responsive poly(N‐isopropylacrylamide‐co‐acrylic acid) (pNIPAM‐co‐AAc) spherical particles of 1.6 μm diameter suspended in aqueous medium. The videos of the trapped particles were digitally processed to track the particle positions as a function of time. From these measurements lateral trap stiffness for pNIPAM‐co‐AAc particles was determined as a function of trap power and temperature using Equipartition and Boltzmann Statistics methods. Both the methods gave similar results and the value for the trap stiffness at 25 °C with trapping laser power of 33 mW was estimated to be 0.14±0.01 μN/m. Since the optical trap stiffness depends on particle size and refractive index which vary as a function of temperature the variation of the measured optical trap stiffness as a function of temperature could be used to determine the volume phase transition of the thermo‐responsive microgel particles. The results should also be useful in investigating the interaction between pNIPAM‐co‐AAc particles trapped in different lattice configurations that can be generated using HOT.