# Water Jacket Systems for Temperature Control of Petri Dish Cell Culture Chambers
## 3 Results and Discussion
### Comparison of Temperature Gradients across a Petri Dish under Different Stable Conditions  
Figure 6 compares the magnitude of the temperature gradient along the radial axis of the Petri dish (\(z=1\) mm) for all stable cases presented in Figures 2-5. We also investigated the effect of microscope stage, washer material, and Petri dish lid on the thermal characteristics of the water jacket system, as presented in Figures S12-S14. It should be noted that a lid is usually inserted on the Petri dish in order to maintain sterility of the sample, which is a crucial condition for cell culture. A quick look at this figure reveals that the following five parameters, including the (i) flow rate of the water recirculating through the water jacket, (ii) Petri dish material, (iii) placing of the Petri dish on a microscope stage, (iv) washer material, and (v) inserting a lid onto the Petri dish, did not change the temperature gradient significantly. In contrast, the following seven parameters significantly changed the temperature gradient across the sample, as summarized below:  
_Water inlet temperature:_ Increase of water inlet temperature from 25 to 45 \({}^{\circ}\)C increased the temperature gradient from 1 to 4.4 \({}^{\circ}\)C, as presented in Figure 3b and Figure S3. Interestingly, similar temperature gradients were obtained for water inlet temperatures of 15 \({}^{\circ}\)C (5 \({}^{\circ}\)C colder than ambient) and 25 \({}^{\circ}\)C (5 \({}^{\circ}\)C hotter than ambient), which clearly indicates the dependence of the temperature gradient on the ambient temperature.  
_Ambient temperature:_ Decrease of ambient temperature from 24 to 16 \({}^{\circ}\)C increased the temperature gradient from 2.4 to 4 \({}^{\circ}\)C, as presented in Figure 3c and Figure S4.  
_Free convection coefficient:_ Increase of free convection coefficient occurred at the free surfaces of the system from 5 to 20 W/m\({}^{2}\)K increased the temperature gradient from 2.1 to 4.3 \({}^{\circ}\)C, as presented in Figure 3d and Figure S5.  
_Petri dish diameter:_ Decrease of the Petri dish diameter from 45 to 15 mm (50% filled in all cases) decreased the temperature gradient from 5.1 to 0.9 \({}^{\circ}\)C, as presented in Figure 4a and Figure S6.  
_Sample volume:_ Increase of the volume of the sample inside the Petri dish from 25% to 75% (for a 35 mm diameter Petri dish) decreased the temperature gradient from 4.2 to 2.9 \({}^{\circ}\)C, as presented in Figure 4b and Figure S7. A slightly higher temperature gradient was obtained when increasing the sample volume from 75% to 100%, which was attributed to higher convective loss at the top surface of the Petri dish due to higher thermal conductivity of water compared to air.  
_Water jacket geometry:_ A semi-circular water jacket led to an asymmetric temperature profile with a temperature gradient of 12 \({}^{\circ}\)C, the highest value obtained in our investigations, as presented in Figure 5b.  
_Integrated heater:_ Integrating a heater underneath the Petri dish led to a uniform temperature across the sample with a temperature gradient of 0.2 \({}^{\circ}\)C, the lowest in our investigations, as presented in Figure 4d and Figure S9.