AC ELECTROTHERMAL MICROFLUIDIC TWEEZERS: CHARACTERIZATION AND APPLICATIONS
Creators: name: Gupta, Kshitiz
Publisher: name: Purdue University Graduate School
Year: 2022
Subjects: Mechanical engineering not elsewhere classified

description: Microfluidics has established itself as a key technology in a wide range of fields including pharmaceuticals, point-of-care diagnostics, thermal management, and space technology. Most of these applications involve manipulation of small quantities (micro – nanoliters) of fluids and various particles or biological cells suspended in them. These platforms employ mechanical, thermal, acoustic, magnetic, optical, electric and many other means for creating particle and fluid motion. Many biological applications require handling cells that are vulnerable to getting damaged if proper physiological conditions are not maintained or if excessive force is applied on them. The non-invasive nature of optical and electrical micro-manipulation techniques such as rapid electrokinetic patterning (REP) has proven to be of great importance in such applications. These techniques enable handling, transportation, sorting and arrangement of fragile synthetic micro/nanoparticles and biological cells without compromising their structure and surface properties. REP is a recently developed micro-manipulation tool that employs optically induced electrothermal vortices to create custom flow patterns. Particle suspensions are entrained in these vortices and are trapped on an electrode surface through AC electrokinetic mechanisms. This work focuses on characterizing a REP trap and discusses its potential applications in handling biological cells. Polystyrene microparticles are confined in a REP trap and a MATLAB program is used to track their motion inside the trap. The tracked particle trajectory reveals that the potential energy of the trapped particle is parabolic and hence the trap is Hookean in nature. The trap is modelled as a spring-mass system and the stiffness coefficient of that system is found to be of the order of 10<sup>-15</sup> N/μm. The origin of the restoring force in the spring-mass model is found to be the drag force created by the electrothermal vortex. The ability to exert ultra-small forces in a stable trap enables REP to be used in various non-invasive particle manipulation applications. The transient nature of REP is studied using numerical modeling and particle image velocimetry (PIV) analysis of a vortex created by a moving laser spot. A numerical model suggests that custom-shaped steady state REP vortices can be created via superposition of multiple axisymmetric circular shaped vortices. However, the method of superposition cannot be extended to transient traps and a more involved 3D model is required to simulate them. The laser spot is scanned back-and-forth in a line with different speeds to create transient REP vortices. The PIV analysis, in agreement with the numerical model, shows that the location of the moving vortex is undiscernible at high speeds. Moreover, the circular shaped vortex is stretched out into a line when the laser scanning frequencies are more than 15 Hz. The particle-electrode attraction force, which entraps the particles at the electrode surface, is characterized using particle diffusometry (PD) and defocusing particle tracking. PD is used to measure the diffusion coefficient of polystyrene particles under different electric field parameters near an electrode surface. It is found that the particle diffusivity decreases with a decrease in the electric field frequency from 150 – 30 kHz and with an increase in the applied voltage from 4 – 8 V<sub>pp</sub>. A MATLAB program is used to track the number of in-focus particles and their distance from the electrode surface. A histogram of the particles’ distance from the electrode surface shows an increase in the particle concentration near the electrode at low frequencies (30 – 60 kHz). These observations suggest that the average height of an entrapped particle decreases with a decrease in applied field frequency and an increase in applied voltage. This suggests that the attractive trapping force is significant at 30 kHz but diminishes at around 150 kHz. Salt and sugar-based isotonic media used for cell suspensions pose several challenges for electrokinetic mechanisms such as REP. Various solutions to overcome these challenges for bio-manipulation applications are discussed in this work. The presence of DC offset in the AC electric field is found to enhance particle entrapment in sugar-based media. The effect of DC offset on trapping performance in bio-relevant media is assessed by measuring the stability of the REP trap. This work also shows entrapment and manipulation of Mice pancreatic cancer cells (KPC2) suspended in the sugar-based isotonic media using REP. The biological applications of the REP technology are highly promising, but they have not yet been well-explored. This work lays the foundation of understanding how REP can be operated in high osmolarity media for bio-manipulation applications.