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  • Open Access

Stressing and probing with light: A thermo-optical investigation of soft solids

Rafael Gervasone, Arianna Erba, Roberto Piazza, and Stefano Buzzaccaro*

  • Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta,” Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

  • *Contact author: stefano.buzzaccaro@polimi.it
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Phys. Rev. Research 8, 043112 – Published 30 October, 2026

DOI: https://doi.org/10.1103/5m7y-k8kc

Abstract

Internal stresses in arrested soft solids can be generated not only by mechanical loading but also by spatially nonuniform temperature fields, which impose incompatible thermal strains and activate coupled thermoelastic and poroelastic responses. Here, we introduce an all-optical stress-and-probe protocol that combines localized near-infrared photothermal forcing with space-resolved dynamic light scattering. The setup provides (1) thermal-lensing readout of the evolving refractive-index field and (2) photon-correlation imaging of local dynamics, complemented by speckle-based velocimetry to reconstruct displacement and velocity fields. We apply the method to two contrasting soft solids. In a thermoresponsive physical gel, continuous irradiation produces a slowly strengthening lensing signal that persists after the pump is switched off; refractive-index reconstruction indicates contributions beyond purely thermo-optic effects, consistent with irreversible microstructural reorganization. Concomitantly, correlation functions reveal a long-lived modification of the relaxation dynamics, supporting a scenario where constrained thermoelastic stresses imprint residual strain and drive postexcitation stress relaxation. In an ultraweak polystyrene colloidal gel with nearly temperature-independent constitutive properties, the sample deformation is driven by a buoyancy-driven convection, whose magnitude scales with the thermal expansivity of water and increases with pump power, crossing from largely elastic recovery to partial irreversibility. Overall, this optical platform enables controlled access to ultralow, spatially distributed thermal stresses and offers a route to study stress transmission, plasticity, and cyclic photothermal fatigue in soft solids.

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Phys. Rev. Research 8, 043112– Published 30 October, 2026
  • Received 27 April 2026
  • Accepted 5 September 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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