Dr. Giuliano Zanchetta, University of Milan, Italy

The relevance of soft materials for our daily life is nested in their multiple length scales and rich mechanical behaviour, viscoelastic response and flow properties. Active microrheology techniques, based on the application of an external force to an embedded microparticle, can provide important insights on the correlation between local mechanical properties and macroscopic response. However, established techniques like optical tweezers are limited to rather weak forces, while magnetic tweezers can apply large yet inhomogeneous forces, and are limited to low frequencies.
A recently developed technique, based on a microfluidic chip and optical scattering forces acting on a microbead [1], can be implemented for both creep and oscillatory experiments, easily reaching hundreds of pN and providing access to a wide range of nonlinear phenomena. The design consists of two counter-propagating infrared beams carried into a microchannel by two facing waveguides, realized by femtosecond laser fabrication in a glass substrate. The relative intensity of the two beams can be temporally tuned by a shutter, providing a step function, or by an integrated Mach-Zehnder optical modulator oscillating between 0.01 and 10 Hz.
We exploit the potential of the optofluidic micro-rheometer for the investigation of a fibre network displaying yield stress, i.e. a transition from solid-like to liquid-like response above a threshold applied stress [2]. We explore the microscale yielding, finding quantitative agreement with bulk measurements, and characterize the deformation of the portion of material surrounding the microbead.
Furthermore, we study the linear and nonlinear viscoelastic response of hydrogels based on the self-assembly of DNA strands [1]. The exquisite control over the structure and interactions of building blocks allows to tune the phase behaviour, connectivity and stress-bearing properties of the resulting networks, and to embed fluorescent strain reporters [3].
[1] Nava et al., Soft Matter 14, 3288 (2018)
[2] Zanchetta et al., Colloid Polym. Sci. 296, 1379 (2018)
[3] Zanchetta, COCIS 40, 1 (2019)