## 3 Results and Discussion
### Effects of Different Flow Rates on Concentration Distribution of Chemotactic Bacteria  
Toluene in the oil phase is a chemoattractant of chemotactic bacteria. The diffusion and dissolution of toluene from the oil phase to the water phase determine the distribution of chemotactic bacteria. Taking chip 1 as an example, the concentration distribution of chemotactic bacteria at different flow rates was analyzed. The specific method to find out the concentration distribution of chemotactic bacteria at the oil-water two-phase interface is to describe the contour when the phase field variable \(\varphi=0\), and then obtain the concentration of the chemotactic bacteria. Figure 8: Residual oil volume versus time in the high and low permeable zones for Chip 1 at different inlet flow rates. LP represents low permeable zone, HP represents high permeable zone. Figure 9: Distribution of toluene concentration (left) and bacteria concentration at different flow rates at the proposed steady state. The scale bar is 1 mm; (**a**,**c**,**e**,**g**) are the distribution of toluene concentration at the proposed steady state with inlet velocities of 1.13, 2.26, 4.52 and 6.78 m/d, respectively; (**b**,**d**,**f**,**h**) are the distribution of chemotactic bacteria concentration at the proposed steady state with inlet velocities of 1.13, 2.26, 4.52 and 6.78 m/d, respectively. In this figure, the numbers to the left of the colored bars are in mol/m\({}^{3}\). The concentration of the chemotactic bacteria at the interface between oil and water was 1.1-1.5 mol/m\({}^{3}\) at the front end of the oil-water interface in the hypotonic zone and 1.05-1.1 mol/m\({}^{3}\) at the oil-water interface in the hypertonic zone at an inlet flow rate of 1.13 m/d. The concentration of chemotactic bacteria at the junction of the hypotonic and hypertonic zones was 1.2-1.8 mol/m\({}^{3}\) at the front end of the hypotonic zone and 1.1-1.3 mol/m\({}^{3}\) at the oil-water interface in the hypertonic zone, at an inlet flow rate of 2.26 m/d. The concentration of chemotactic bacteria at the oil-water interface in the front end was 1.2-1.8 mol/m\({}^{3}\), and the concentration of chemotactic bacteria at the oil-water interface in the hypertonic zone was 1.1-1.3 mol/m\({}^{3}\). At an inlet flow rate of 4.52 m/d, the concentration of chemotactic bacteria at the oil-water interface in the front end at the junction of the hypotonic zone and the hypertonic zone was 1.3-1.5 mol/m\({}^{3}\), and the concentration of chemotactic bacteria at the oil-water interface in the hypertonic zone was 1.2-1.6 mol/m\({}^{3}\). At an inlet flow rate of 6.78 m/d, the chemotactic bacteria were mainly enriched at the oil-water interface in the hypertonic zone, with concentrations of up to 1.3-2 mol/m\({}^{3}\). The curve of the concentration of chemotactic bacteria at a certain point at the interface with time is shown below. It can be seen from Figure 10 that the concentration of chemotactic bacteria changes with time at a certain point at the oil-water two-phase interface at different flow rates. When the water phase carrying chemotactic bacteria flows through this point, the chemotactic bacteria are rapidly enriched. The greater the flow rate is, the earlier the time is to reach the steady state, and the concentration of chemotactic bacteria at the phase interface decreases with the decrease of the flow rate at the steady state. This is because, when the oil phase is displaced, the toluene in the oil dissolves into the water phase to form a concentration transition zone, and the higher the flow rate is, the steeper the concentration gradient is. Therefore, the enrichment degree of chemotactic bacteria is higher at higher flow rates. One interesting finding is the transient peak before the steady state in each curve.