Question 4: Liquids in Space Shuttle

Solution

In a space shuttle environment, effective gravity $g \approx 0$. The equilibrium shape of a liquid is determined entirely by surface tension forces (contact angle).

• Water on Glass: Water wets glass (Contact angle $\theta \lt 90^\circ$). It seeks to maximize the contact area with the glass surface. Therefore, the water will spread out and coat the entire inner surface of the hollow sphere, leaving a bubble of air in the center.
• Mercury on Glass: Mercury does not wet glass (Contact angle $\theta > 90^\circ$). It seeks to minimize contact with the glass surface. Therefore, it will pull itself together into a single spherical drop floating inside the sphere to minimize its surface energy.