# Extending Cooling Rate Performance of Fast Scanning Chip Calorimetry by Liquid Droplet Cooling
## 3 Results  
### Liquid Cooling of Empty Sensors  
The cooling performance is significantly improved by using liquid droplet cooling and synchronous switching off the heater, as illustrated in Figure 6a on the example of an empty XI395 sensor cooled with chilled ethanol from ca. 620 K to room temperature. Because the ballistic cooling follows an exponential law, Figure 6b shows the data plotted on a logarithmic scale. It is visible that in the entire temperature range down to the surrounding temperature, the liquid droplet cooling gives two orders of magnitude increase in the cooling rate. The insert of Figure 6a shows the work of a trigger system. The upcoming droplet causes a gas movement, which reduces the measured temperature by ca. 3 K. The heater voltage switches off after a 4 K temperature reduction caused by the droplet itself. The initial temperature difference between the blue and the red line is due to slight variations of the experimental conditions during the experiments, e.g., the sensor surface was not thoroughly cleaned after previous droplet cooling. Figure 7 shows the achieved cooling rates for different cooling agents. Results using ethanol, chilled ethanol and cold-spray are shown and compared to gas cooling (air). The other cooling media did not improve the cooling rate compared to the shown liquids. Room temperature ethanol is most useful to achieve a maximum cooling rate of 4,000,000 K/s at 500 K, though the cooling rate quickly decays when approaching the base temperature (Figure 7a). If compared in logarithmic scale (Figure 7b), all liquid coolants level at approximately the same order of magnitude of 1,000,000 K/s, but cold liquids are more efficient at temperatures closer to ambient. Cold ethanol has less peak efficiency, since wetting  
Figure 6: Cooling with liquid droplet and synchronous switching off the heater (red) and just switching off the heater in air (black). Temperature versus time (**a**) and cooling rate (**b**) for liquid cooling of XI395 sensor with chilled ethanol. of the surface is delayed because of higher viscosity. Cold-spray allows cooling to lower temperatures, therefore giving better cooling efficiency at 300 K. At high temperatures, cold-spray is the least efficient among the tested three liquids but still allows cooling at rates above 200,000 K/s. This can be explained by the lower boiling temperature of the cold-spray solution and the time required for the liquid to get in contact with the sample, avoiding formation of a gas layer (Leidenfrost phenomenon). The kink at around 270 K is probably due to water condensation, as all experiments were performed at ambient conditions. In a dry gas, the efficiency of the cold-spray at low temperatures may even be better. A water-ethanol mixture was also tested as a cooling agent. The cooling rate around 300 K was significantly increased compared to pure ethanol, similar to chilled ethanol but lower than the cold-spray. It was found that a thin surface layer of water was accumulating on the membrane even when using chilled ethanol due to the condensation of water from the air. This layer was detected by the FSC and makes it challenging to compare the heating curves of the quenched material with reheating curves after gas cooling. Removing surface water would require heating the sensor. Therefore, water-ethanol mixtures were not used eventually, and results are not shown in Figure 7. Other liquids, e.g., acetone or glycerol, were not very useful in the tested temperature range because of the boiling at the sensor surface (Leidenfrost phenomenon) or delayed wetting due to high viscosity, respectively. The evolution of the cooling rate with decreasing temperature is shown in Figure 8a. In the beginning, at high temperature, the cooling rate increases, while it takes until the droplet covers the sample and heat flows from the sample through the whole interface to the droplet.