Micro/nanoswimmers have been intensely investigated for the past decade because of their potential applications in drug delivery [1
], biological sensing [3
], and tissue manipulation [5
]. At the microscale, these devices swim at low Reynolds numbers, where viscous forces dominate over inertia; thus, the fluid flow becomes time reversible. According to the scallop theorem [7
], nonreciprocal motion is required to achieve a net forward thrust at low Reynolds numbers. To generate nonreciprocal swimming strokes in viscosity-dominated environments, many existing micro/nanoswimmers utilize helical structures or flexible bodies.
Rigid helical micro/nanoswimmers mimic the swimming motion of bacteria such as Escherichia coli
and can be obtained using a number of top-down and bottom-up techniques, including self-scrolling [8
], 3D direct laser writing [9
], and nucleic acid manipulation [10
]. Microswimmers with flexible bodies are analogous to the flagella of sperm, which can generate propulsion by propagating nonreciprocal traveling waves down the flexible flagellum [11
]. So far, most of the flexible microswimmers have been constructed with rigid segments that are connected by soft junctions. However, the complex or costly fabrication process of these two kinds of microswimmers limits their further application, despite their swimming properties [12
]. Chemically driven propulsion with hydrogen peroxide (H2
) has the fastest swimming speed among microswimmers thus far, but this requires specific chemical environments for actuation [13
]. Other systems such as acoustic- [15
], light- [16
], and electrostatic-activated [17
] microswimmers have also attracted significant attention; here, we focused on magnetically driven propellers.
Apart from helical or flexible body microswimmers, studies have demonstrated that rigid achiral microswimmers can also swim at low Reynolds numbers under rotating magnetic fields, such as particle-based microswimmers [18
], which was the first reported achiral microswimmer, in part to re-examine the minimal geometrical requirements for designing microswimmers. In a follow-up study, Cheang and Kim [19
] thoroughly discussed the feasibility of fabricating achiral microswimmers that, because of their 2D simplistic geometries, could be fabricated at low cost using high-throughput lithography techniques. This was later corroborated in experiments with planar microswimmers [20
]. Furthermore, the swimming properties of the achiral microswimmers were extensively investigated, with the conclusion that achiral planar shapes are nearly optimal propellers [21
]. Similar achiral structures with asymmetric arms have also been used to obtain imbalanced forces and induced torques in other systems [22
Aside from the above-mentioned microswimmers that are designed for swimming in bulk fluid, magnetically actuated rolling microrobots are actively being studied because of their simplicity. For example, a dumbbell-like microrobot can be used for cargo transportation using the microvortices it generates [24
]. Similarly, the microvortices generated by the rotational microparticles have been used for trapping objects such as live bacteria [25
]. These two examples of cargo transportation using rolling robots, which took advantage of microvortices for localized fluid trapping, illustrate the importance and potential applications of studying the low Reynolds number hydrodynamics of microswimmers. However, unlike microswimmers, these devices are limited to locomotion on a surface [15
Unlike their chiral counterparts (e.g. rigid helices), achiral microswimmers have not been thoroughly investigated, specifically their hydrodynamic properties are largely unknown. Understanding the hydrodynamics of a microswimmer can lead to better control and practical applications. Even though numerical simulations were conducted to study the hydrodynamics of achiral microswimmers in bulk fluid [27
] and near boundaries [29
], so far there have been no experimental reports on the flows produced by these swimmers.
The microparticle image velocimetry (µ-PIV) technique can be used to measure the velocity flow field of a microswimmer by measuring the moving patterns of laser-excited tracer particles within a fluid in sequential frames. For the past few years, µ-PIV had been used to study the flows of a number of different microswimmers. For example, µ-PIV was used to further the understanding of the swimming behavior of microorganisms [30
], as well as a microorganism-based microswimmer [33
], while stereoscopic µ-PIV measurements were used to confirm the asymmetric dynamic motion of artificial cilia that can generate 3D asymmetric dipole vortices [34
]. For helical microswimmers, [35
] studied the thrust force and efficiency of helical microswimmers using µ-PIV with both micro- and macroscale models. Recently, Mart´ınez-Aranda et al. studied the flow dynamics around several microswimmers prototypes, such as spherical, elliptical, and cylindrical shapes, in non-Newtonian fluids using µ-PIV in order to simulate the hydrodynamics of microswimmers inside a human vessel [37
]. Moreover, a µ-PIV study was conducted to investigate the fluid flow generated by microspheres, which are widely used for micromixing and pumping, mask-free colloidal patterning, and as microrobots in the form of rolling robots [13
]. Even though the µ-PIV technique has been used for investigating chiral and flexible microswimmers, there is still a lack of literature on achiral microswimmers.
Here we report on the µ-PIV characterization of achiral L-shaped microswimmers fabricated using photolithography and thin-film deposition, which allowed us to visualize the flow field and obtain quantitative data for validation. The results presented herein will provide a better understanding of the hydrodynamics of achiral shapes, which will aid in the future application of achiral microswimmers for microfluidic tasks.