Microscopic Engine Powered by Critical Demixing

Abstract: By converting energy into mechanical work, engines play a central role in most biological and technological processes. In particular, within the current trend towards the development of nanoscience and nanotechnology, microscopic engines have been attracting an ever-increasing interest. On the one hand, there has been a quest to understand how biological molecular motors work. On the other hand, several approaches have been proposed to realize artificial microscopic engines, which have been powered by the transfer of light momentum, by external magnetic fields, by in situ chemical reactions, or by the energy flow between hot and cold heat reservoirs, in scaled-down versions of macroscopic heat engines. Here, we experimentally demonstrate a microscopic engine powered by the local reversible demixing of a critical mixture. We show that, when an absorbing microsphere is optically trapped by a focused laser beam in a sub-critical mixture, it is set into rotation around the optical axis of the beam because of the emergence of diffusiophoretic propulsion; this behavior can be controlled by adjusting the optical power, the temperature, and the criticality of the mixture. Given its simplicity, this microscopic engine provides a powerful tool to power micro- and nanodevices. Furthermore, since many biological systems are tuned near criticality, this mechanism might already be at work within living organisms, for example in proteins and in cellular membranes.

• The paper presents a novel method for powering a microscopic engine using reversible critical demixing of a water–2,6-lutidine mixture.
• It employs optical trapping and video microscopy, complemented by Langevin dynamics, to validate diffusiophoretic propulsion under varied laser and thermal settings.
• The findings offer promising applications in microfluidics and biomedical devices by introducing innovative pathways for energy conversion at nanoscales.

Microscopic Engine Powered by Critical Demixing

The study of microscopic engines, particularly in the physics community, often hinges on understanding the intersection of energy conversion at nano and micro scales and the inherent stochastic behavior due to thermal fluctuations. "Microscopic Engine Powered by Critical Demixing" presents a notable addition to this domain, demonstrating a novel method of propulsion through the manipulation of a critical mixture's demixing properties.

Summary of Findings

The paper experimentally reveals a microscopic engine driven by the local reversible demixing of a critical mixture, specifically a binary mixture of water and 2,6-lutidine. Here, an absorbing microsphere is held in place by an optical trap. The microsphere, which comprises silica and iron oxide inclusions, effectively harnesses the surrounding mixture's criticality to generate a controlled rotational motion via diffusiophoretic propulsion, thereby functioning as a microscopic engine.

Several parameters influence this system's operation, including laser power, ambient temperature, and the criticality of the mixture. The researchers found that, at strategic combinations of these variables, the microsphere could achieve stable rotational motion driven by the synergistic interplay of thermal gradients and concentration fluctuations.

Numerical and Experimental Analysis

The experimental setup integrates a near-infrared laser for optical trapping and video microscopy for precise tracking of particle behavior. The critical findings reveal that the diffusiophoretic motion dominates when the microsphere is near the trap focus, with rotational motion emerging as the dominant force at optimal conditions—a significant contribution to the understanding of non-equilibrium thermodynamic systems.

Experimental observations are validated by numerical simulations employing Langevin dynamics. These simulations consider optical forces, temperature profiles arising from laser absorption, and the diffusiophoretic effects due to temperature-induced concentration gradients. The consistency between simulation and experiment underscores the robustness of the proposed engine mechanism.

Implications and Future Work

The introduction of phase transition mechanics as a propulsive engine marks a considerable development in the design of micro and nanoscale devices. Practically, this suggests potential applications in environments where traditional energy conversion methods are impractical. For instance, its implementation in biomedical contexts, leveraging the benign and low-energy nature of the process, could lead to novel surgical tools or drug delivery mechanisms, especially in microfluidic or environments that mimic biological systems.

Theoretically, this research opens new discussions about utilizing phase transition phenomena in energy systems and draws attention to the unexplored opportunities of critical phenomena in natural and synthetic systems. Understanding the phase behavior around critical points could potentially unlock new ways to design more efficient thermal management systems at micro scales.

Conclusion

The paper provides a comprehensive view of a novel microscopic engine reliant on phase transition, demonstrating innovative principles that could drive future developments in micro-engineering and biotechnological applications. As the field evolves, future research might explore alternative critical mixtures or leverage advances in nanotechnology to enhance engine performance and efficiency. Through examining the intricacies of such systems in greater detail, researchers could further bridge the gap between theoretical physics and real-world technological applications.