where 𝜈 obs [Hz] is defined as 𝑥̇Δ𝑥 ⁄ , 𝑉 is the displaced fluid volume [m 3 ], Δ𝑥 is the body's length [m] in the direction of transport (e.g., sphere diameter), and 𝐴 s is the interfacial surface area [m 2 ] (Fig. ). Meanwhile, the friction factor 𝑁 f is defined here as