 Adhesive-cell rolling threshold

For a weakly adhering elastic sphere, balancing elastic contact energy $G{a}^5/R^2$ against adhesion $J{a}^2$ gives contact radius $a\sim (J{R}^2/G{)}^{1/3}$. If peeling the rear contact dissipates work of order $J$ per unit area, its rolling moment is $M_r\sim JaR$. Balancing this with a fluid shear moment $\eta \dot{\gamma }{R}^3$ gives the displayed estimate. A perfectly reversible elastic contact has no such net rolling-energy barrier; adhesive hysteresis or bond kinetics must justify a finite threshold, and the corresponding dissipative adhesion energy replaces $J$.