Hidden Structural Control of Solvent Transport under Soft Jamming

Abstract

Transport in soft jammed materials is often described as fluid motion through a fixed structure, leading naturally to capillary based descriptions. This picture appears particularly appropriate in strongly jammed systems, where structural rearrangements are suppressed and little visible motion is observed. Here we investigate solvent transport in foam and show that this intuition fails to capture key aspects of the transport process. By directly observing both liquid penetration and bubble motion under controlled boundary conditions, we demonstrate that solvent transport is strongly influenced by the mechanical response of the foam structure, even though the intrinsic imbibition relative to the foam matrix remains purely capillary-driven. In closed systems, the jammed structure resists penetration and leads to a pronounced slowdown that cannot be accounted for by purely capillary descriptions. In contrast, in open systems, collective bubble motion accompanies solvent invasion, resulting in an apparent acceleration of transport. These results indicate that the lack of structural motion does not guarantee a purely capillary description of transport. Our findings reveal a boundary controlled coupling between flow and structure, and highlight the need to reconsider transport processes in soft jammed systems, including foams, dense colloids, and biological tissues.

Figures (8)

• Figure 1: Experimental setup. (a) Schematic diagonal view of the foam imbibition cell. A foam layer is confined between two acrylic plates with a spacer of thickness 1.0 mm. A dyed surfactant solution is brought into contact with one side of the foam. (b) Top view showing the boundary conditions. The injection side is either open or covered by a semipermeable membrane that blocks bubble motion while allowing solvent transport. The opposite side is either open or sealed with an acrylic plate, defining fully open, half-open, and closed geometries.
• Figure 2: Boundary-condition-dependent imbibition dynamics. (a) Time evolution of the liquid penetration distance $x_{\mathrm{liq}}$ for a foam with mean bubble diameter $d = 0.22$ mm and liquid fraction $\phi = 0.10$. Data are shown for fully open (circles), half-open (squares), and closed (triangles) boundary conditions. Solid lines are power-law fits. (b) Imbibition exponent $\alpha_1$, defined by $x_{\mathrm{liq}} \sim t^{\alpha_1}$, as a function of the liquid fraction $\phi$ for different boundary conditions.
• Figure 3: Size-dependent imbibition dynamics. Time evolution of the liquid penetration distance $x_{\mathrm{liq}}$ for foams with mean bubble diameters $d = 0.22$ mm (opened) and $0.15$ mm (filled). Data are shown for closed (circles) and half-open (squares) boundary conditions. The foam composed of larger bubbles exhibits faster imbibition.
• Figure 4: Visual observation of foam motion during imbibition in the half-open geometry. Representative images of the foam before (a) and during (b) liquid imbibition. The dyed liquid penetrates from the left. The dashed line indicates the initial position of the right edge of the foam. As imbibition proceeds, the foam matrix translates in the direction of liquid invasion.
• Figure 5: Differential image analysis of foam motion. Differential image obtained from intensity fields over a time interval $\delta t = 0.5$ s during imbibition in the half-open geometry. The solid line marks the position of the right edge of the foam matrix, and the dashed line indicates the liquid penetration front. Heterogeneous intensity variations appear only within the imbibed region, whereas the foam ahead of the front undergoes an almost rigid translation.