Optoelectronic tweezers (OET) is an opto-electro-fluidic micromanipulation technology that uses light-induced dielectrophoresis (DEP) for touch-free actuation of micro-scale objects in physical, chemical and biomedical studies. In this work, we introduce a new method to evaluate the behavior of particles trapped in OET traps and used this technique to study their escape mechanism. Particles experiencing negative DEP were made to move in a circular path on a microscope stage, such that the particles’ velocities and trajectories could be observed under different conditions. At high velocities, particles were observed to escape the trap vertically (into the suspending medium) before settling back onto the surface. Three-dimensional numerical simulations of electrical field distribution indicated that the vertical displacement phenomenon occurs when the particle experiences the strongest DEP force at the boundary of the light pattern, lifting the trapped particle to a region where viscous drag exceeds the local horizontal DEP force, thereby forcing it to escape OET confinement. Similar ‘hopping’ phenomena were also observed for cells and particles of different sizes. We propose that the escape mechanism clarified in this work is a general one for objects manipulated by negative DEP in an OET trap, which will be important to consider in the future design of OET-enabled micromanipulation tools for a wide range of applications.
Light patterned dielectrophoresis or optoelectronic tweezers (OET) has been proved to be an effective micromanipulation technology for cell separation, cell sorting and control of cell interactions. Apart from being useful for cell biology experiments, the capability of moving small objects accurately also makes OET an attractive technology for other micromanipulation applications. In particular, OET has the potential to be used for efficiently and accurately assembling small optoelectronic/electronic components into circuits. This approach could produce a step change in the size of the smallest components that are routinely assembled; down from the current smallest standard component size of 400×200 μm (0402 metric) to components a few microns across and even nanostructured components. In this work, we have demonstrated the use of OET to manipulate conductive silver nanowires into different patterns. The silver nanowires (typical diameter: 60 nm; typical length: 10 μm) were suspended in a 15 mS/m solution of KCL in water and manipulated by positive dielectrophoresis force generated by OET. A proof-of-concept demonstration was also made to prove the feasibility of using OET to manipulate silver nanowires to form a 150-μm-long conductive path between two isolated electrodes. It can be seen that the resistance between two electrodes was effectively brought down to around 700 Ω after the silver nanowires were assembled and the solution evaporated. Future work in this area will focus on increasing the conductivity of these tracks, encapsulating the assembled silver nanowires to prevent silver oxidation and provide mechanical protection, which can be achieved via 3D printing and inkjet printing technology.