Manipulation of Fluids and Particles under an Electrothermal Field

• Main Authors: Yu-Liang Chen, 陳昱良
• Other Authors: Hong-Ren Jiang
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/4mfwm2

博士 === 國立臺灣大學 === 應用力學研究所 === 106 === Electrically and thermally based manipulations of fluids or particles has been a technological trend used in lab-on-a-chip applications in last decades. In particular, several studies report that it could generate local inhomogeneities of electric propertiy on fluids and particles by a thermal gradient and further drive the fluids and particles by an electric field acting these inhomogeneities, such as electrothermal flow. Compared with the manipualtion of fluids and particles under a single physical field, the manipulation under an electrothermal field reveals a wide range flow pattern and fast velocity. Based on these features, we plan to apply an electrothermal coupled field to manipulate fluids, particles and even a single asymmetric particle. In this thesis, we demonstrate a functional rotating electrothermal technique for rapidly concentrating and sorting a large number of particles on the microchip by the combination of particle dielectrophoresis (DEP) and inward rotating electrothermal (RET) flows. Different kinds of particles can be attracted (positive DEP) to or repelled (negative DEP) from the electrode edges, and then the n-DEP responsive particles are further concentrated at the heated region by RET flows. This multi-field technique can be operated in salt solutions and at higher frequency without external flow pressure. It can avoid the electrokinetic phenomena at low frequency to improve manipulation accuracy for lab-on-chip applications. To further apply the electrothermal field to drive a single asysmmetric particle (Janus particle), the characters of polarization of particles under an electric field must be understood in advance. In this thesis, the polarization of metal-coated Janus particles is characterized by electrorotation (EROT) measurements. The rotational direction of Janus particles following or countering the direction of the rotating electric field are observed depending on the field frequency and thickness of metallic coating. The comparison of Janus, metallic, and dielectric particles reveals that the hemispherical coating reduces the screening effect and promotes polarization, thereby exhibiting a higher characteristic frequency. We propose that there is a special length scale introduced by metallic coating in the polarization of Janus particle. Subsequently, the electrothermal field is used to drive a singlemetal-coated Janus particle. By applying an AC electric field on self-thermophoretic Janus particles in a defocused laser beam, the velocity becomes faster than that of usual self-thermophoretic Janus particles. We propose that the enhancement of self-thermophoresis could be explained by the induced zeta potential resulting from applying an AC electric field. Based on the experimental results, the manipulation technique under an electrothermal field is more effective than the technique under a single physical field. It would be useful if the properties of fluids and particles can be dynamically tuned by one field and driven by the other field. The functional manipulation technique under an electrothermal field may give a new way to power and control the motion of particles and fluids in a microchip.