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Laser-induced heating for the experimental study of critical Casimir forces with optical trapping
by Ignacio A. Martinez, Artyom Petrosyan, Sergio Ciliberto
This Submission thread is now published as
Submission summary
| Authors (as registered SciPost users): | Ignacio A. Martinez |
| Submission information | |
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| Preprint Link: | scipost_202311_00009v1 (pdf) |
| Date accepted: | 2023-11-13 |
| Date submitted: | 2023-11-07 10:26 |
| Submitted by: | Martinez, Ignacio A. |
| Submitted to: | SciPost Physics |
| Ontological classification | |
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| Academic field: | Physics |
| Specialties: |
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| Approach: | Experimental |
Abstract
Critical Casimir interactions represent a perfect example of bath-induced forces at mesoscales. These forces may have a relevant role in the living systems as well as a role in the design of nanomachines fueled by environmental fluctuations. Since the thermal fluctuations are enhanced in the vicinity of a demixing point of a second-order phase transition, we can modulate the magnitude and range of these Casimir-like forces by slight changes in the temperature. Here, we consider two optical trapped colloidal beads inside a binary mixture. The Casimir interaction is controlled by warming the mixture by laser-induced heating, whose local application ensures high reproducibility. Once this two-particle system is warmed, the critical behavior of different observables allows the system to become its self-thermometer. We use this experimental scheme for analyzing the energetics of a critical colloidal system under a non-equilibrium-driven protocol. We quantify how the injected work can be dissipated to the environment as heat or stored as free energy. Indeed, our system allows us to use the fluctuation theorems framework for analyzing the performance of this critically driven toy model. Our work paves the way for future experimental studies on the non-equilibrium features of bath-induced forces and the design of critically driven nanosystems.
Author comments upon resubmission
Dear Editor, please, find below our discussion with the referees. First of all, we want to acknowledge both referees for their kind and swift comments. The manuscript has been changed according to their suggestion.
- Variations of the temperature will also change the order parameter profile around the particle. Due to the difference in dielectric properties of the two phases (which is exploited by the authors for the measurement of temperature) this may also influence the optical trapping. Can the authors comment on that?
We acknowledge the referee for pointing out it. Indeed, the trap stiffness is a function of the ratio between the refractive index of the media and the refractive index of the colloidal particle. The refractive index scales with the media density, which changes with a small universal exponent, hence its influence can be neglected. Nevertheless, the measurement relying on light scattering hinges on either the gradient or the Laplacian of the refractive index field. Consequently, the variations are significantly amplified by these derivatives.
- Did the authors also consider vertical changes in the particle position during their protocol?
We disregarded the second-order effects of the hydrodynamic tensor, particularly the orthogonal coupling. In future studies, it will be important to consider the cross-motion to assess potential alterations in the drag term, specifically for microrheological investigations of the sample.
- It would be helpful, the authors could discuss the differences between their engine and a colloidal Carnot or Stirling engine. My impression is, that apart from the technical realization, there are large similarities. In both cases the available phase volume of the colloid is periodically changed which leads to the extraction of work.
Our system differs significantly from conventional Carnot or Stirling cycles due to the absence of two thermal reservoirs from which useful work is extracted. Our primary objective is to scrutinize energy current fluctuations when the driving force originates from a thermal bath. Interestingly, our two-state system can be reinterpreted as a Szilard engine if we engineer the balance between its two potential wells. This augmentation introduces the intriguing dimension of the thermodynamics of information. Furthermore, leveraging the unique properties of critical Casimir forces, we can envision an information-emerging system where the role of information, such as a sequence, can be toggled on and off with minor temperature adjustments.
List of changes
Requested changes
1. Variations of the temperature will also change the order parameter profile around the particle. Due to the difference in dielectric properties of the two phases (which is exploited by the authors for the measurement of temperature) this may also influence the optical trapping. Can the authors comment on that?
We added the following sentence:
As in previous studies of CCF with optical trapping, we did not report any change in the trap stiffness along the protocol. The index of refraction scales as the density, i.e. Delta n/n \propto \Delta \rho/\rho. However, the measures based on light scattering are based either on the gradient or on the Laplacian of the index of refraction field. Therefore the fluctuations are strongly enhanced by the derivatives.
2. Did the authors also consider vertical changes in the particle position during their protocol?
We added the following sentence:
Our particle tracking is restricted to the XY plane while we only analyse the trajectory in the x-axis. We neglect the cross coupling between the two axis (x and y) since this perturbation is second order in the Rotne-Prager diffusion tensor.
3. It would be helpful, the authors could discuss the differences between their engine and a colloidal Carnot or Stirling engine. My impression is, that apart from the technical realization, there are large similarities. In both cases the available phase volume of the colloid is periodically changed which leads to the extraction of work.
We added the following paragraph:
As future applications, it is pertinent to highlight two potential approaches for harnessing useful work from the depicted schematic. Firstly, the system's dual-equilibrium configuration exhibits a notable resemblance to the colloidal Szilard engine. By effectively manipulating the probabilities associated with each equilibrium position, one can intentionally disrupt the system's ergodic behavior, thereby facilitating the extraction of valuable work within the framework of thermodynamics of information. Secondly, a more advantageous avenue emerges. By carefully adjusting the temperature differentials during the forward and reverse processes, it becomes possible to readily access and extract the stored energy in the form of free energy. This rudimentary model aligns with the foundational principles that underlie the impact of critical interactions on biological systems.
Published as SciPost Phys. 15, 247 (2023)