s are the results of the linear fittings for time before the boat started moving.](sup.2_2.eps){width="7cm"}

Next, we investigated $r^2$ at time $t$, where $r$ was the longest distance between the edge and the center of the region with the camphor layer, shown as the white region in Fig. \[diffusion\]. The closed circles, open squares and closed triangles in Fig. \[diffusion2\](a) show the data for $p=0~\%$ ($\mu=$ 0.92 mPa$\cdot$s) for water, and $p=40~\%$ ($\mu=4.03$ mPa$\cdot$s) and $p=70~\%$ ($\mu=25.80$ mPa$\cdot$s) for the glycerol-water solution, respectively. Let us focus on the data for $p=0$. The trend of the data changed around at $t\sim0.2$ s, which almost corresponded to the time when the camphor boat began to move as shown in Fig. \[diffusion\]. As we needed the effective diffusion coefficient of the camphor, we measured $r^2$ in the time range in which the camphor boat did not move. Figure \[diffusion2\](b) is an expanded figure for small $t$, i.e. time without the boat motion. When the camphor boat stayed at a certain position, $r^2$ increased linearly with time. Linear fittings are shown as solid lines, where fitting was executed for the region 0 s $ <t<$ 0.13 s for closed circles. The gradients of these solid lines provide the effective diffusion coefficients $D$ of the camphor molecules on the glycerol-water solution. The effective diffusion coefficient on the water was obtained at $180~(\pm~20)$ mm$^{2}$/s. A previous paper [@Kitahata2] reported that the effective diffusion coefficient $D$ in a numerical study almost agreed with the value for $D$ measured with this method. Thus, we consider that this method is reasonable for the measurement of $D$. The gradient of the solid line decreases with an increase in the glycerol concentration. Figure \[diffusion3\] shows the relationship between $p$ and $D$, and the tendency that $D$ decreases with an increase in $p$ was confirmed.

![\[diffusion3\] Effective diffusion coefficient $D$ against glycerol concentrations $p$ of the base solutions. E