Recent progress in field-directed assembly of colloids in an evaporating droplet

Evaporation-driven self-assembly of colloidal particles in a sessile drop can construct simple structures in various scenarios. However, the non-equilibrium self-assembly process, dominated by internal capillary flow and solute- or thermal-induced Marangoni flow, cannot guarantee the precise positioning of the particles. One can exert forces on individual suspended particles by introducing external field manipulation, which enables the construction of precise deposits and even 3D structures. Here, we review recent advances in field-directed assembly techniques, including acoustics, optics, electronics, and magnetics, where micro- and nano-particles are directly guided into targeted positions to form functional structures. These techniques are fundamental in analytical chemistry for their ability to achieve precise particle positioning, which is essential for developing sensitive and specific analytical methods. We first discussed the forces generated by these external fields and then summarized the directed assembly of colloidal particles, including its benefits and disadvantages for analytical chemistry applications. Furthermore, we examine the challenges and potential solutions for controlling colloidal particles and explore future research directions to advance the field of analytical chemistry further.