多顆微粒子之介電泳交越頻率的量測與模擬分析

Abstract

近年來介電泳現象被廣泛用於生物晶片及實驗室晶片技術等，藉由改變外加交流電場頻率、溶液種類與導電度等，即可操控或是分類溶液中不同粒徑或材質之微粒子，而在探討介電泳現象時，微粒子置於溶液中形成之電雙層不可被忽略，其存在對微粒子導電度有相當大之影響，因其能大幅提高粒子整體導電度。 本研究在實驗部分利用光鉗系統搭配鎖相放大器過濾掉雜訊以提升雜訊比，去量測單、雙顆球形粒子之交越頻率，透過訊號產生器輸入一調幅頻率，以微粒子振動相位變化量測交越頻率，並且在量測時利用壓電控制器控制載台將被光鉗捕捉之粒子移動至適當位置，避免電滲、電熱等電液動流場干擾，影響微粒子運動導致模擬與量測結果不符，在到達交越頻率時，鎖相放大器會出現明顯的相位差，此方法能夠準確量測交越頻率。 而本研究在模擬部分利用不同於以往常見的計算方法去分析微粒子介電泳性質，過去通常假設電雙層之表面電導為定值，不過表面電導實際上會因離子吸附作用而改變，故本研究根據Zhao等人發展出的體積積分法為參考，將模擬與量測數據擬合得到初始表面電荷密度，針對不同溶液導電度、粒子粒徑及表面官能基去分析交越頻率，並將電滲、電熱等電液動流場效應納入考慮再與量測結果比較，以及透過模型設定不同幾何形狀擴展此方法之應用，探討多顆粒子與單顆粒子之交越頻率關係，以及在有無表面官能基的條件下，粒子之體積與表面之電雙層涵蓋面積變化率去解釋交越頻率的變化趨勢以及其他不同幾何參數條件會對交越頻率產生什麼影響。 經過量測與模擬結果得知，交越頻率與粒子電荷吸附量、史吞層導電度、粒子的粒徑形狀以及表面官能基相關，並發現交越頻率與粒徑有特定比例關係，而依據不同表面官能基條件，透過兩粒子粒徑重疊的比例發現交越頻率會隨著體積與表面積變化率的不同影響力而會有所差異，以及在串聯多顆粒子情況下，交越頻率不僅會與粒子數n-0.095成正比，且此正比關係不受粒子性質、大小與溶液導電度影響，因此可藉由已知的單顆球形粒子交越頻率估算，由此單顆球形粒子串聯而成的多顆粒子，交越頻率為多少。

In recent years, the phenomenon of dielectrophoresis (DEP) has been widely implemented to Lab-on-chip technology. By simply changing the frequency of the external AC electric field, the type of medium and medium conductivity, we are able to manipulate micro-particles with various sizes and dielectric properties. When considering the DEP, one cannot ignore the effect of electric double layer (EDL) that forms around the micro-particle, the presence of EDL will make the conductivity of the particles rise drastically. In this study, we use the optical tweezers, amplitude modulation (AM) input and lock-in amplifier to filter out the noise and improve the signal/noise ratio (S/N ratio) in order to detect the vibration signal phase of the single and two-sphere particle. The noise is not coherent, so it cannot pass though the lock-in amplifier. In the measurement, the piezoelectric controller is used to control the stage to move the particles captured by the optical tweezers to an appropriate position to avoid electrohydrodynamic flow such as electro-osmosis and electrothermal effect, which affects the movement of the micro-particles and causes the simulation and measurement results to be inconsistent. At the crossover frequency, the particle motion experiences a sharp change of phase shift by 180°, relative to the phase of the amplitude modulation frequency. Therefore, we can obtain the precise crossover frequency. The simulation uses different calculation methods different from the previous method to analyze the dielectrophoretic properties of the micro particles. In the past, it was assumed that the surface conductance of the particle was constant, but the surface conductance actually changed due to ion adsorption. Therefore, the study refers to the volumetric integration method from Zhao et al. After the simulation and measurement data were fitted to obtain the initial surface charge density, we analyzed the crossover frequency under the condition of different particle size, functional group and charge adsorption and took electrohydrodynamic flow effect into account and compared with the measurement results. In addition, the crossover frequency relationship between multi-particles and single particle was discussed by setting different geometric shapes of model. And we discuss the volume of the particles and the surface area of electric double layer change rate in the presence or absence of surface functional groups to explain the trend of the crossover frequency and the influence of other different geometric parameters on the crossover frequency. From the measurement and simulation results, the crossover frequency is related to the charge adsorption amount, the electric double layer conductivity, the particle size and shape and the functional group of the particle. It is found that the crossover frequency has a specific proportional relationship with the particle size. According to the different functional group conditions, the crossover frequency will be different according to the difference of the volume and surface area change rate. And in the case of multi-particles, the crossover frequency is not only proportional to the number of particles n-0.095, and this proportional relationship is not affected by the particle properties, size and conductivity of the solution. Therefore, it is possible to estimate the crossover frequency of multi-particle by the known crossover frequency of single particle.