## 3 Results and Discussion  
The temperature measurement method using the pNIPAM solution should be evaluated by comparing it with the thermocouple data. The gold film temperature is adjusted with 4 cases using a heating plate (case 1: 26 \({}^{\circ}\)C, case 2: 27 \({}^{\circ}\)C, case 3: 28 \({}^{\circ}\)C, and case 4: 29 \({}^{\circ}\)C). As shown in Figure 4, the gold film temperature is measured using the thermocouple and pNIPAM solution measurement method. \(T_{1,TC}\) is the gold film temperature measured by the thermocouple, and \(T_{1,p}\) is the gold film temperature estimated by the measurement method using pNIPAM solution. The pNIPAM solution temperature is averaged over the area within the red dotted circle in Figure 4. The present method is validated with the maximum difference of 0.14 \({}^{\circ}\)C for case 3, which lies within the measurement uncertainty range of the thermocouple. DI-water droplet (22 \({}^{\circ}\)C, 1 \(\mu\)L) is deposited on the second gold film above the pNIPAM layer to examine the solid-liquid interface temperature (see Figure 1b). The incidence angle of the light is fixed at 75\({}^{\circ}\). Figure 5 shows the side view of the droplet and the corresponding SPR image at the surface temperature, \(T_{s}\), of 29.0 \({}^{\circ}\)C. The contact diameter of the droplets, \(D_{w}\), is 1.79 \(\pm\) 0.1 mm, and the contact angle is 77.3 \(\pm\) 1.5\({}^{\circ}\). In the SPR image, the dark color is the pNIPAM solution, and the bright color is ambient. The lower the temperature, the lower the reflectance of the pNIPAM solution showing a relatively dark color. The magnified region of the blue dotted area is depicted in Figure 5c. The SPR image  
Figure 4: Comparison of temperature between pNIPAM solution measurement, \(T_{1,p}\), and thermocouple, \(T_{1,TC}\). Figure 3: (a) Refractive index variation of pNIPAM solution with the reflectance, (b) temperature variation according to the refractive index of pNIPAM solution. has a dark color at the center of the droplet due to the decrease of the temperature, and it gradually brightens toward the contact line region. Temperature distributions over a pNIPAM solution are estimated through the reflectance obtained by the corresponding SPR intensity. Figure 6a shows the reflectance of the pNIPAM solution along with line AB (in Figure 5c) right after a droplet deposition. The initial reflectance of the pNIPAM solution before the droplet deposition is about 0.1746, 0.2106, and 0.2680 for the surface temperature, \(T_{s}\), of 27.0, 28.0, and 29.0 \({}^{\circ}\)C, respectively. A DI-water is located at \(-\)895 \(\upmu\)m to 895 \(\upmu\)m with a diameter of 1790 \(\upmu\)m; a minimum reflectance is observed near the center (\(r=0\)\(\upmu\)m) for all cases. Reflectance varies relatively large for the surface temperature of 29.0 \({}^{\circ}\)C. However, other cases have a small change of reflectance at the location of the droplet because the refractive index of the pNIPAM solution drastically deviates with the increase of the temperature, as in Figure 3b. The temperature of the pNIPAM solution, \(T_{1p}\), is obtained by using Equations (2) and (3). A measurable temperature range is 24.3 \({}^{\circ}\)C through 29.3 \({}^{\circ}\)C considering the thermo-sensitivity of pNIPAM under the present experimental setup. The lowest temperature of pNIPAM solution is observed at the center of the droplet for all cases--the temperature increases toward the contact line of the droplet. As the surface temperature decreases, the variation of the pNIPAM solution temperature decreases. Moreover, the pNIPAM solution temperature is lower than the surface temperature at the outer region of the droplet due to the heat transfer through the transverse direction of the multi-layer. The solid-liquid interface temperature of the droplet is estimated by solving the one-dimension heat conduction problem with the pNIPAM solution temperature as a boundary condition.