Biophysical Society Thematic Meeting | Riga 2026

Active and Responsive Soft Matter: From Biological to Engineered Systems

Poster Abstracts

2-POS Board 2 QUANTITATIVE PHASE IMAGING OF PULSATING EPITHELIA: GIANT DENSITY FLUCTUATIONS, WAVES, AND THE ROLE OF CELL MOTILITY

Dag Kristian Dysthe 1 ; Luiza Angheluta-Bauer 1 ; Silja B Låstad 1 ; Nigar Abbasova 1,2 ; 1 University of Oslo, Physics, Oslo, Norway 2 Copenhagen University, Niels Bohr Institute, Copenhagen, Denmark

Spontaneous oscillations of cell area, height, and density are common in confluent epithelial monolayers in vitro. These can be considered biological examples of pulsating active matter which is a class of systems where each unit expands and contracts and the synchronizing interaction between the units give rise to emergent collective behaviour. Recent theories of pulsating active matter predict a range of behaviour rationalized by topological defects [1-3]. Our recent development of quantitative phase imaging (QPI) of confluent monolayers [4,5] allows us to probe directly the density field, the oscillating phase and the associated topological defects. From timelapse QPI we obtain the demeaned density field $\delta\rho(\mathbf r,t)= \rho-\langle \rho\rangle_t$ and extract the oscillation phase $\psi$ and amplitude $A$ by a temporal Hilbert transform, $Z=A\,e^{i\psi}=\mathcal H_t[\delta \rho]$. From $\psi$ we obtain the instantaneous frequency $\omega=\partial_t\psi$, the phase-gradient orientation $\chi=\arg(\nabla\psi)$, and its associated topological charge density $\rho_\chi = \epsilon_{\alpha\beta} (\partial_\alpha \cos\chi)(\partial_\beta \sin\chi)$. The defects of $\psi$ sit on the interfaces between contracting ($\delta\rho>0$) and expanding ($\delta\rho<0$) regions as predicted for the pulsating phase [2,3], and the frequency and orientation fields reveal the wave structure of the pulsation. Quantitative comparison between the integrated continuous charge and discrete winding counts show excellent agreement. QPI thus resolves all the fields required by the theories at single-pixel precision. The general phenomenology also agrees with Tang et al [6] that used cell tracking to calculate velocity divergence to approximate the same fields. The static structure factor of the QPI density field scales as $S(q)\!\sim\!q^{-2}$ at sub-cellular wavevectors which signifies \emph{giant density fluctuations} and the anti-hyperuniform branch of the phase diagram of Li et al [3]. The synchronization order parameter of Li et al, $M=|\langle e^{i\psi}\rangle_{\mathbf r}|$ and the per-cell defect density $\rho_t$ are linearly anti-correlated, with $M$ increasing and $\rho_t$ decreasing with cell density as predicted in Li et al's model.$S(q)$ also exhibits a Brillouin-like peak whose frequency $\Omega_B$ and half-width $\Gamma_B/2$ both scale linearly with $q$ indicating a linear dispersion with a propagation speed $c_s\!\approx\!15~\mu\mathrm{m\,h^{-1}}$. Such propagating density waves are absent from the fixed cell pulsating-matter theories, whose phase dynamics are diffusive. Their existence, together with the fact that the cells visibly migrate, motivates an extension of the theory to motile cells. We show that an active stress $\sigma^a\propto\cos\psi$ can account for an additional, advective part of the dynamics although intrinsic topological dynamics, not migration, dominate the pulsations.[1] Zhang, Y., \& Fodor, É. (2023). Pulsating Active Matter. \textit{Phys. Rev. Lett.}, 131, 238302.[2] Banerjee, T., et al. Hydrodynamics of pulsating active liquids. \textit{arXiv:2407.19955} (2025).[3] Z. Li, Q. Lei, and Y. Ma, Fluidization and anomalous density fluctuations in 2D Voronoi cell tissues withpulsating activity,' \emph{Proc. Natl. Acad. Sci. U.S.A.} \textbf{122}, e2421518122 (2025).[4] L{\aa}stad, S. B., Abbasova, N., Combriat, T. \& Dysthe, D.K., Quantitative phase imaging of the dynamics of epithelial monolayers, SPIE Proceedings 138610A, (2026)[5] L{\aa}stad, S. B., Abbasova, N., Combriat, T. \& Dysthe, D.K., Three Dimensional Dynamics of Epithelial Monolayers, bioRxiv 2026.03.10.710903 (2026).[6] W. Tang, M. R. Nejad, A. F. Pegoraro, L. Mahadevan, and M. Guo, Collective synchrony in confluent, pulsatile epithelia, arXiv:2507.16772 (2025)

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