Temporal evolution of viscoelasticity of soft colloid laden air-water interface: a multiple mode microrheology study

Mechanical properties of particle laden interfaces is crucial for various applications. For water droplets containing soft microgel particles, passive microrheology studies have revealed that the dynamically varying surface area of the evaporating drop results in a viscous to viscoelastic transition along the plane of the interface. However, the behaviour of the medium orthogonal to the interface has been elusive to study using passive microrheology techniques. In this work, we employ optical tweezers and birefringent probe particles to extract the direction-resolved viscoelastic properties of the particle-laden interface. By using special types of birefringent tracer particles, we detect not only the in-plane translational mode but also the out-of-plane translational (perpendicular to the interface) and rotational modes. We first compare different passive methods of probing the viscoelasticity of the microgel laden interface of sessile drop and then study the modes perpendicular to the interface and the out-of-plane rotational mode using optical tweezers based passive microrheology. The viscoelasticity of the interface using two different methods, i.e., multiple-particle tracking passive microrheology using video microscopy and by trapping birefringent tracer particles in optical tweezers, relying on different models are studied and found to exhibit comparable trends. Interestingly, the mode orthogonal to the interface and the rotational mode also show the viscous to viscoelastic transition as the droplet evaporates, but with lesser viscoelasticity during the same evaporation time than the in-plane mode.