# Extending Cooling Rate Performance of Fast Scanning Chip Calorimetry by Liquid Droplet Cooling
## 4 Discussion  
Liquid droplet cooling with the sensor at room temperature conditions enhances the cooling rate compared to gas cooling by about two orders of magnitude. For the empty sensor XI395, comparable with the UFH1 sensor of Mettler Toledo, cooling rates of 5,000,000 K/s are achievable. The possibilities of liquid droplet cooling are further illustrated by experiments with indium, HDPE and PBN. HDPE quenching was done with varying the liquid droplet size, and the crystallization peak maximum temperatures versus cooling rate were compared to data measured earlier on different calorimeters (Figure 11) [31]. The range of cooling rates from 1000 to 1,000,000 K/s is covered by ultra-fast scanning calorimetry, using a combination of a small sensor (XI392, heated area 8 \(\times\) 6 \(\upmu\)m\({}^{2}\)), loaded with an ultra-small sample (ca. 1 ng) and using helium gas at 83 K as the heat transfer fluid. The new liquid droplet cooling technique (red stars) allows for measuring solidification of a 40 ng HDPE sample with the sensor at room temperature at rates up to 100,000 K/s. The crystallization temperatures are compared for the same cooling rate for a 40\(\times\) smaller sample and with \(T_{\rm t}\) = 83 K. The measured values of the crystallization temperatures of the HDPE at different cooling rates coincide with previously measured values within the error of the measurements. This confirms that the temperature of the thermopile represents the temperature of the sample, and there are no significant gradients evident in the large system at these rates. A different situation is seen with the cooling of the indium particle (Figure 9b). The crystallization onset temperature of indium shifts unexpectedly towards higher temperatures with an increasing cooling rate. Crystallization of the indium particle occurs at the peak of the cooling performance. Due to comparable thermal contact resistances between sample, droplet and thermopiles, strong temperature gradients arise in the system, with details schematically shown in Figure 12. This is different for polymer samples, as polymers wet the sensor surface and have better thermal contact with the sensor. Thus, the measured higher onset temperature of the tiny indium sample indicates that the real temperature of the sample is lower than the thermopile temperature, which means the actual cooling rate is even faster than measured by the thermopile. Figure 10: Poly (butylene 2,6-naphthalene) (PBN) crystallization enthalpy as a function of cooling rate on the XI395 sensor under liquid nitrogen conditions (**a**). The maximum possible cooling rate at \(T_{\rm g}\) when using the Mettler Toledo UFS1 sensor at room temperature is indicated by the vertical green line. The vertical red line corresponds to the critical cooling rate avoiding crystallization on cooling, and the vertical blue line provides the maximum cooling rate at \(T_{\rm g}\) achieved by liquid droplet cooling. Fast scanning chip calorimetry (FSC)-reheating curves of PBN prepared as indicated in the legend, with the measurements done using the UFS1 in a self-designed calorimeter (**b**). Further experiments with larger liquid droplets support this idea. The apparent increase in the crystallization onset temperature in the cooling curve is seen in Figure 9b, red and pink curves. As shown in Figure 8a, the cooling rate can potentially be controlled using different droplet sizes. In all shown measurements, the droplet was significantly larger (about 100\(\times\)) than the heated area of the sensor and the sample, but it is still possible to realize a rough control of the cooling rate. Further investigations in this direction are required. Additionally, limitations of the liquid droplet cooling by reducing the cooling rate caused by intense boiling of the cooling liquid, especially on sensors with the larger heated area, need to be prevented, e.g., by choosing a more suitable coolant.