FIG. 4. Capillary forces increase aggregate internal pressure.

(A) A schematic describing the decomposition of forces and pressures at $A=A_0$ on deformable substrates. (B) Effective surface tension as a function of substrate stiffness when measured from monolayer stress (red), and active elasto-capillary deformations (blue) $(\mathrm{N}=10$ samples per data point). Effective elasto-capillary length as a function of substrate stiffness calculated from experimental data (solid grey) and under assumption that surface tension stays constant (dashed grey). $l_c$ and $E_c$ are the critical elasto-capillary length and critical substrate stiffness, respectively, where the transition between modes of migration occurs. (C) Aggregate spreading rate as a function of the aggregate surface tension on a 0.7 kPa substrate $(\mathrm{N}=27)$. Surface tension as a function of aggregate size (inset). (D) Pressure on top and bottom section of an aggregate at 0.7 kPa and 8.6 kPa (inset). The dashed black line represents the minimum pressure, $\left(P_{min}=135.8\pm 4.4\mathrm{P}\mathrm{a}\right)$, required to overcome traction. The dashed blue line is basal pressure, $\left(P_0=109.6\pm 6.2\mathrm{P}\mathrm{a}\right)$, for largest aggregates. Error bars are error propagated from measurements of $l_{max}$ and $z_{max}$. (E) Mean pressure on top and bottom sections as a function of substrate stiffness ($\mathrm{N}=9$ for 0.7 and 8.6 kPa each). Error bars are mean ± standard deviation.