*2.3. Physical Property Measurements*

The density of the solutions was measured using a pycnometer with an electric balance. However, the density effect can be ignored when using a horizontal Hele-Shaw cell and when the gap between the cells is small enough.

Another important property is interfacial tension between the displacing and displaced solutions. The interfacial tension was measured by a spinning drop tensiometer, SITE100 from KRÜSS, Germany. The interfacial tension measurement was carried out in a rotating horizontal capillary filled with a high-density liquid. A drop of low-density fluid was placed inside the liquid. Because the rotation of the horizontal capillary creates a centrifugal force, which is against the interfacial tension of the fluids, the drop elongates, making the drop radius smaller, until it reaches a certain radius where the interfacial tension and centrifugal force are balanced. This device calculates the interfacial tension of the system using the following equation:

$$
\gamma = \frac{r^3 \omega^2 \Delta \rho}{4},
\tag{2}
$$

where *r* (m) is the drop radius, ω (rad/s) is the angular frequency of rotation, and Δρ (kg/m3) is the density difference between the two fluids. Here, ω was fixed at 6000 rpm (628 rad/s).

The other important physical property is the viscosity of the solutions. The viscosity was measured using an AR-G2 rheometer from TA Instruments, New Castle, DE, USA. We measured shear viscosity to investigate the viscosity itself and confirmed whether our solutions were Newtonian using a cone–plate-type sensor.
