Different precipitation scenarios produced by counter-diffusion in capillaries. (a) Liesegang bands formed by minerals. The experimental setup is represented by a reservoir of highly concentrated anions and a capillary tube filled with a homogenous gel containing cations. Left panel: graphical representation of the band formation along the capillary. The two counter-diffusing reagents are gradually consumed upon precipitation of their ion product, which generates rings of increased spacing as a function of time. (b) Protein crystallization by counter-diffusion in capillaries. The capillary contains a small volume of gelled protein solution at millimolar concentration and the reservoir contains a large volume of precipitant at molar concentration. Initially the precipitant penetrates the gel and accumulates at its highest concentration near the interface, where it triggers protein precipitation. This achieves the highest supersaturation level and ratio, which is illustrated with a blue line on the graph. As the precipitant equilibrates against the capillary, lower concentrations are reached gradually inside the gel (purple, green and red lines, respectively) and both the levels and the ratios of supersaturation decrease in time and space, which is favourable for nucleation and crystal growth. The steady state is reached in approximately two weeks. (c) Liesegang-like pattern for TollN6–VLR in the presence of malonate. Capillary 1 was left to equilibrate against 2.4 M sodium malonate pH 7.0; capillary 2 against 3.4 M sodium malonate pH 7.0; and capillary 3 against 0.15 M I3C, 2.89 M sodium malonate pH 7.0. Photographs of the lower section of each capillary have been taken after two weeks. (d) Graph representing the relative distances ξn of TollN6–VLR bands as a function of the number of bands in each capillary. (e) Determination of the spacing coefficient P, represented by the slope of the linear regression (P = 0.97 for TollN6–VLR). The graph shows the relative distances of successive bands ξn+1 represented as a function of ξn, from all TollN6–VLR capillaries. (f) TollN6–VLR does not obey the width law. The graph shows the relative distance of band n (ξn) represented as a function of its width (wn).