Challenges and attempts to make intelligent microswimmers

The study of microswimmers’ behavior, including their self-propulsion, interactions with the environment, and collective phenomena, has received significant attention over the past few decades due to its importance for various biological and medical applications. Microswimmers can easily access micro-fluidic channels and manipulate microscopic entities, enabling them to perform sophisticated tasks as untethered mobile microrobots inside the human body or microsize devices. Thanks to the advancements in micro/nano-technologies, a variety of synthetic and biohybrid microrobots have been designed and fabricated. Nevertheless, a key challenge arises: how to guide the microrobots to navigate through complex fluid environments and perform specific tasks. The model-free reinforcement learning (RL) technique appears to be a promising approach to address this problem. In this review article, we will first illustrate the complexities that microswimmers may face in realistic biological fluid environments. Subsequently, we will present recent experimental advancements in fabricating intelligent microswimmers using physical intelligence and biohybrid techniques. We then introduce several popular RL algorithms and summarize the recent progress for RL-powered microswimmers. Finally, the limitations and perspectives of the current studies in this field will be discussed

Competing effects of inertia, sheet elasticity, fluid compressibility, and viscoelasticity on the synchronization of two actuated sheets

Synchronization of two actuated sheets serves as a simple model for the interaction between flagellated microswimmers. Various factors, including inertia, sheet elasticity, and fluid viscoelasticity, have been suggested to facilitate the synchronization of two sheets; however, the importance of different contributions to this process still remains unclear. We perform a systematic investigation of competing effects of inertia, sheet elasticity, fluid compressibility, and viscoelasticity on the synchronization of two sheets. Characteristic time s for the synchronization caused by inertial effects is inversely proportional to sheet Reynolds number Re, such that s∝Re−1 with ω being the wave frequency. Synchronization toward stable in-phase or opposite-phase configuration of two sheets is determined by the competition of inertial effects, sheet elasticity, fluid compressibility, and viscoelasticity. Interestingly, fluid viscoelasticity results in strong synchronization forces for large beating amplitudes and Deborah numbers De > 1, which dominates over other factors and favors the in-phase configuration. Therefore, our results show that fluid viscoelasticity can dramatically enhance synchronization of microswimmers. Our investigation deciphers the importance of different competing effects for the synchronization of two actuated sheets, leading to a better understanding of interactions between microswimmers and their collective behavior

Molecular dynamics simulation of a nanoscale feedback-free fluidic oscillator

We present a molecular dynamics simulation study of a feedback-free fluidic oscillator model. Using the molecular dynamics simulations, it is demonstrated that the oscillation can be self-induced and sustained in a large range of flow rate and two very different jet directions. The oscillation mechanism of the nanoscale fluidic oscillator is physically similar to that in macroscale in which the dome vortex plays a crucial role. The thermal fluctuation is not significant enough to submerged the effect of hydrodynamics in the nanoscale feedback-free fluidic oscillator. The linear relationship between the oscillation frequency and the flow rate revealed by macroscopic experiments was also found in our simulations. Two of the three oscillation regimes found in macroscopic studies are shown to be able to be reproduced in our simulation. Our results show that molecular dynamics simulation is fully capable of studying the complicated flow in a feedback-free fluidic oscillator

Molecular Dynamics Study on the Mechanism of Nanoscale Jet Instability Reaching Supercritical Conditions

This paper investigates the characteristics of a nitrogen jet (the thermodynamic conditions ranging from subcritical to supercritical) ejected into a supercritical nitrogen environment using the molecular dynamics (MD) simulation method. The thermodynamic properties of nitrogen obtained by molecular dynamics show good agreement with the Soave-Redlich-Kwong (SRK) equation of state (EOS). The agreement provides validation for this nitrogen molecular model. The molecular dynamics simulation of homogeneous nitrogen spray is carried out in different thermodynamic conditions from subcritical to supercritical, and a spatio-temporal evolution of the nitrogen spray is obtained. The interface of the nitrogen spray is determined at the point where the concentration of ejected fluid component reaches 50%, since the supercritical jet has no obvious vapor-liquid interface. A stability analysis of the transcritical jets shows that the disturbance growth rate of the shear layer coincides very well with the classical theoretical result at subcritical region. In the supercritical region, however, the growth rate obtained by molecular dynamics deviates from the theoretical result

Exploring the link between coffee matrix microstructure and flow properties using combined X-ray microtomography and smoothed particle hydrodynamics simulations

Abstract Coffee extraction involves many complex physical and transport processes extremely difficult to model. Among the many factors that will affect the final quality of coffee, the microstructure of the coffee matrix is one of the most critical ones. In this article, we use X-ray micro-computed (microCT) technique to capture the microscopic details of coffee matrices at particle-level and perform fluid dynamics simulation based on the smoothed particle hydrodynamics method (SPH) with the 3D reconstructured data. Information like flow permeability and tortuosity of the matrices can be therefore obtained from our simulation. We found that inertial effects can be quite significant at the normal pressure gradient conditions typical for espresso brewing, and can provide an explanation for the inconsistency of permeability measurements seen in the literature. Several types of coffee powder are further examined, revealing their distinct microscopic details and resulting flow features. By comparing the microCT images of pre- and post-extraction coffee matrices, it is found that a decreasing porosity profile (from the bottom-outlet to the top-inlet) always develops after extraction. This counterintuitive phenomenon can be explained using a pressure-dependent erosion model proposed in our prior work. Our results reveal not only some important hydrodynamic mechanisms of coffee extraction, but also show that microCT scan can provide useful microscopic details for coffee extraction modelling. MicroCT scan establishes the basis for a data-driven numerical framework to explore the link between coffee powder microstructure and extraction dynamics, which is the prerequisite to study the time evolution of both volatile and non-volatile organic compounds and then the flavour profile of coffee brews