在這篇論文中，我們藉由光學鑷子的特性來探討細菌地毯產生的力在微流道裡是如何透過流體傳播的問題。在低雷諾數的狀況下，細菌透過鞭毛的運動達成自身的生物趨向性。在此鞭毛攪動流體的過程中，此運動透過流體的傳播會造成鄰近細菌運動行為上的改變。而透過流體的交互作用，群聚的細菌或鞭毛也有可能在空間或時間上產生大尺度的同步運動。細菌鞭毛的同步運動所產生的力透過流體傳播所產生的影響即是我們這篇論文所研究的重點。我們量測的細菌地毯是由一群高濃度單鞭毛細菌(VIO5和NMB136)在載玻片上以沉積的方式所形成的二維排列。VIO5(CW+CCW+flicking)和NMB136(CCW)的鞭毛轉速可以藉由緩衝液中的鈉離子濃度來做為調整，大大提升了實驗的複雜性。在此實驗中，我們藉由調控光學鑷子對粒子的束縛強度，可以觀察出粒子在流場下的脫離強度，進而推測出對應的力的訊號(~pN)。藉由光學鑷子的空間操控性，我們量測不同轉速與旋轉方向的地毯在垂直方向上的力的空間分佈。我們觀察到VIO5和NMB136所構成的細菌地毯分別產生了推力(遠離地毯表面)和吸力。透過曲線擬合，我們發現兩種地毯鞭毛所造成的力透過流體的傳播在空間上皆滿足1/r3的行為。藉由此流場的分佈，我們可以合理的推測此1/r3的行為有可能來自於緊密細菌排列所導致的邊界效應，進而造成流線局部性的扭曲。在另一方面，我們觀察到當細菌鞭毛轉速到達某個臨界值時，細菌地毯所產生的力會突然驟升，造成一非線性的力的變化。此力隨著鞭毛轉速的驟升很有可能與細菌鞭毛間的群體運動有關。此研究提升了生物在微流道傳輸裡應用的可行性，也提供了一個適當的活體生物與工程之間的合作平台。 We investigate the hydrodynamic spreading force generated from bacterial carpets in microfluidic channel. The bacterial carpets are formed by single-polar flagellated bacteria on poly-lysine treated glass substrate in the closed channel. Two distinctive bacterium strains (NMB136 and VIO5) with Na+ driven flagellar motors are employed respectively. We adopt a particle assay based on optical tweezers, which is capable of conducting the resolution of force measurement up to pN. Through tuning down the trapping power, one can qualitatively characterize the external force from the trajectories of particles. Subsequently, varying heights (10 to 20 μm) and different Na+ concentrations (30 to 300 mM) are performed. As a result, attractive and repulsive forces are respectively detected above 10 μm from bacterial carpets due to the boundary effect and propelling forces from collective flagellar rotations. We found that a collapse of normalized curves into r-3 curve, which implies the appearance of a quadruple-like behavior. Meanwhile, it is found that the force strengths increase abruptly upon the threshold rotational rate, showing a nonlinear transition with a counter-intuitive physical description. Synchronization of flagella is the one of possible candidates that provides more appropriate explanation. In summary, we demonstrate a force spreading measurement above bacterial carpet in microfluidic channel. The boundary effects and collective rotations characterize the resultant force patterns. Moreover, the onset of the flagella synchronization probably accounts for the transition-like behavior, which implies more complex mechanism involved in this system. Our work presents a novel microfluidic manipulation and provides suitable platforms for microbial-fluid interface engineering.
