Microrobots are anticipated to operate within confined environments, such as the blood vessels of the human body, porous tissue matrices, or confined microfluidic chips. The degree of confinement imposed by the surrounding boundaries plays a crucial role in the locomotion of microrobots, as it leads to increased hydrodynamic interactions with the outer boundaries of the environments (Acemoglu & Yesilyurt, 2014; Fang, Ham, Qiao, & Tao, 2020; Martínez-Pedrero et al., 2021; Temel & Yesilyurt, 2015). Among the various microrobotic platforms, magnetic surface microrollers have demonstrated immense potential in diverse biomedical applications, including cargo delivery through blood flow navigation and potential utilization in lab-on-a-chip systems (Alapan, Bozuyuk, Erkoc, Karacakol, & Sitti, 2020; Bozuyuk, Aghakhani, et al., 2022; Bozuyuk, Alapan, Aghakhani, Yunusa, & Sitti, 2021; Bozuyuk, Ozturk, & Sitti, 2023b; Bozuyuk, Suadiye, et al., 2022; Chamolly, Lauga, & Tottori, 2020; Demirörs et al., 2021; Dou, Tzelios, Livitz, & Bishop, 2021; Driscoll et al., 2017; Lee et al., 2023; Martín-Roca et al., 2022; Qi et al., 2021; Rogowski et al., 2021; Shanko, Ceelen, Wang, van de Burgt, & den Toonder, 2021; van der Wee et al., 2022; Wu et al., 2022). However, their translational locomotion has proven to be challenging in circular confinements, such as cylindrical channels, as they reverse their translational locomotion direction due to rotational flows generated by a single microroller (Bozuyuk, Aghakhani, et al., 2022). Therefore, robust locomotion in such confinements cannot be achieved due to fundamental hydrodynamic barriers when the microroller operates individually within such confinements.
Swarming of microrobots offers significant advantages in terms of locomotion and enhanced imaging contrast (Jin et al., 2023; Kaiser, Snezhko, & Aranson, 2017; Sun et al., 2022; Sun et al., 2021; Xie et al., 2019; Yang, Wang, Jin, Du, & Zhang, 2022; Yu, Wang, Du, Wang, & Zhang, 2018). A single unit in the swarm is also hydrodynamically enhanced by the neighboring units; therefore, swarming could potentially be useful to overcome hydrodynamic barriers encountered by microrobots (Bozuyuk, Aghakhani, et al., 2022; Bozuyuk et al., 2021; Bozuyuk, Ozturk, & Sitti, 2023a). Here, we investigated the effect of swarming microrollers in cylindrical confinements using computational fluid dynamics (CFD) environment. We observed that the swarming microrollers could perform and maintain locomotion in their desired direction, while a single microroller exhibits reverse locomotion. Increasing the number of microrollers and reducing the distances between the microrollers within the swarm prove beneficial for enhancing microroller locomotion. Additionally, we conducted a brief experimental demonstration with microrollers in cylindrical tubings to validated our findings from the CFD analyses. In summary, our findings contribute to an enhanced comprehension of locomotion at the micron-scale in confined spaces, providing valuable information for the design and optimization of microrobot locomotion in confined environments.