Research
Transient Dynamics of Phototaxis
The directed movement of microorganisms toward light, known as phototaxis, is a fundamental behavior that enables many unicellular and multicellular photosynthetic microorganisms to survive and grow. In this work, we investigated the time-dependent response of populations of the model phototactic alga Chlamydomonas reinhardtii to changing light conditions.
We found that when the environment changes from dark to light, the response of a dark-adapted isotropic population depends not only on the light intensity but also on the cell number density. In contrast, when the illumination changes from light to dark, the recovery from an aligned state back to an isotropic state is independent of both cell density and light intensity.
Our results show that this transient behavior arises from the coupling between flagellar dynamics, swimming speed, and the incident photon flux. Furthermore, we demonstrate that existing minimal non-interacting models, which successfully describe steady-state phototaxis in Chlamydomonas, reproduce the observed transient dynamics only for dilute cell suspensions.
Related publication: Physical Review Research 7, 033282 (2025)
Response and Recovery
Response
Recovery
Reentrant Efficiency of Phototaxis
Phototaxis is one of the most fundamental stimulus–response behaviors in biology, enabling motile microorganisms to sense light gradients and swim toward a light source. Besides promoting the survival and growth of individual cells, phototaxis plays an important role in aquatic ecosystems and in the performance of algal bioreactors.
In this work, we investigated the phototactic behavior of the unicellular alga Chlamydomonas reinhardtii over a wide range of cell number densities and light intensities using high spatiotemporal video microscopy. Surprisingly, we found that phototactic efficiency reaches a minimum at a well-defined cell density. Beyond this critical density, a population of cells can navigate toward light more efficiently than isolated individual cells.
We showed that this enhancement arises because increasing cell density reduces the swimming speed of individual cells, allowing them to sense the light gradient more effectively. Furthermore, we demonstrated that this steady-state behavior is accurately captured by modeling the cells as active Brownian particles subjected to a density-dependent phototactic torque.
Related publication: Biophysical Journal 117, 1508–1517 (2019)
Cells in dark condition
Cells under light stimulus
Kinetic Spinodal Instabilities in the Mott Transition
Abrupt first-order phase transitions are typically described using classical nucleation theory, where a system transforms by nucleating and growing a new phase. In this work, we investigated the kinetics of the metal–insulator (Mott) transition in vanadium sesquioxide (V2O3) to understand how the dynamics of the transition evolve under nonequilibrium conditions.
We performed the first systematic study of how thermal hysteresis depends on the temperature scanning rate in a correlated electron system. We found that both the supercooling and superheating temperatures shift with the quench rate, and that the dynamic scaling exponent is close to the mean-field prediction of 2/3.
Together with the continuous ordering observed in quench-and-hold experiments, these results indicate departures from classical nucleation theory toward a barrier-free phase ordering governed by critical dynamics. Our findings provide experimental evidence that spinodal-like instabilities can exist in real materials undergoing thermally driven first-order phase transitions, even in the presence of fluctuations.
Related publication: Physical Review Letters 121, 045701 (2018)