SciPost Submission Page
Nonequilibrium Probability Currents in Optically-Driven Colloidal Suspensions
by Samudrajit Thapa, Daniel Zaretzky, Ron Vatash, Grzegorz Gradziuk, Chase Broedersz, Yair Shokef, Yael Roichman
Submission summary
| Authors (as registered SciPost users): | Yael Roichman · Yair Shokef · Samudrajit Thapa |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202407_00026v1 (pdf) |
| Date submitted: | 2024-07-15 11:01 |
| Submitted by: | Thapa, Samudrajit |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Experimental, Computational |
Abstract
In the absence of directional motion it is often hard to recognize athermal fluctuations. Probability currents provide such a measure in terms of the rate at which they enclose area in the reduced phase space. We measure this area enclosing rate for trapped colloidal particles, where only one particle is driven. By combining experiment, theory, and simulation, we single out the effect of the different time scales in the system on the measured probability currents. In this controlled experimental setup, particles interact hydrodynamically. These interactions lead to a strong spatial dependence of the probability currents and to a local influence of athermal agitation. In a multiple-particle system, we show that even when the driving acts only on one particle, probability currents occur between other, non-driven particles. This may have significant implications for the interpretation of fluctuations in biological systems containing elastic networks in addition to a suspending fluid.
Author indications on fulfilling journal expectations
- Provide a novel and synergetic link between different research areas.
- Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
- Detail a groundbreaking theoretical/experimental/computational discovery
- Present a breakthrough on a previously-identified and long-standing research stumbling block
Author comments upon resubmission
Dear SciPost Physics Editors,
We are resubmitting our manuscript, "Nonequilibrium Probability Currents in Optically-Driven Colloidal Suspensions".
We sincerely appreciate the referees' recognition of our manuscript's potential and their acknowledgment of its importance as an "insightful minimal model" with "wider application." We are grateful for their positive and thorough review and attentive reading. In response to their feedback, we have comprehensively revised the manuscript. Below, we address the two main comments raised by the referees. More detailed and specific responses to all comments are provided in our enclosed responses to each of the referee reports. In the revised manuscript we highlight in red the modifications that we introduced.
In response to the referees' primary request, we have restructured the paper to clarify its main message and improve its flow. The revised structure is as follows:
- We begin by emphasizing the significance of the Area Enclosing Rate (AER), a non-invasive measure of probability currents (Battle et al., Science). We explain its crucial role in identifying departures from equilibrium, particularly in systems where direct evidence of dynamical evolution or currents is absent.
- We then highlight that while the primary models developed to understand the relationship between a system's internal activity and the measured AER have focused on systems coupled by elastic springs, many microscopic systems (such as complex fluids and biological matter) are predominantly governed by hydrodynamic interactions.
- Following this introduction, we present a minimal physical model incorporating hydrodynamic interactions. We employ a multifaceted approach, combining experiments, theoretical analysis, and numerical simulations to elucidate the relationship between activity and AER.
- We conclude with a comprehensive discussion of our model's implications and findings.
This restructured format provides a clear progression from the fundamental concept of AER to the novel contributions of our study, emphasizing the importance of hydrodynamic interactions in non-equilibrium systems.
Another request by the referees was to provide a better connection between the AER and entropy production rate (EPR). We have added a full paragraph to the discussion section. In short, the EPR for a linear system is given by the trace of the matrix computed as a product of the AER, the inverse Covariance matrix, and the inverse Diffusion matrix (see Ref. [15] and Eq. 37 in the revised manuscript). The advantage of the AER over the EPR is that it can be computed directly, for any two degrees of freedom, directly from the raw measured trajectories. In contrast, the EPR requires the measurement of the probability of each trajectory and its time-reversed trajectory, two non-trivial, data-intensive measures.
The significance of our work is twofold. First, we provide a crucial extension of the previous work on the AER by studying its behavior when hydrodynamic interactions dominate particle dynamics. These conditions extend the applicability of the AER as an analysis tool for complex active fluids and biological systems. Significantly, we also show that the AER signal decays rapidly with distance in such systems, in contrast to elastically bound systems. This renders the AER a more local measure than expected in hydrodynamic-interaction-dominated systems.
We thank you for the opportunity to improve our work and hope you find the revised manuscript suitable for publication in SciPost Physics.
Sincerely, Samudrajit Thapa (on behalf of the authors)