Table 2.

Example and explanation of hydrodynamic phase distribution anomalies in spiral columns on J-type CCC.^a

Phase system	Physical property of phase system	Anomaly in hydrodynamic phase distribution	Explanation
(i) Heptane/water (1:1); (ii) Heptane/ethyl acetate/methanol/water (1.4:0.6:1:1); (iii) Heptane/ethyl acetate/methanol/water (1.4:2:1:1) see ref. [12]	(i) More hydrophobic, density difference 340 kg/m3, density ratio 1.52, interfacial tension (τ) 52 mN m−1 (ii) More hydrophilic, density difference 230 kg/m3, density ratio 1.33, τ = 6.2 mN m−1 (iii) More hydrophilic, density difference 138 kg/m3, density ratio 1.18, τ = 2.1 mN m−1	For phase system (i), J-type planetary motion makes upper phase move to the head and lower phase to the tail irrespective of head/tail being at periphery or centre. However for phase systems (ii) and (iii), phase distribution results are consistent with the existing head and tail rule.	The solid–liquid friction force in phase system (i) is able to overcome the effect of tangential centrifugal force when the two forces counteract, and hence determines phase distribution outcome. High density difference ensures high solid–liquid friction force, as coordinated by high normal centrifugal force. High interfacial tension ensures that the solid–liquid friction force exerts preferentially to lower phase.
PEG1000-K2HPO3 (18% w/w–18% w/w) ATPS (this work)	Very hydrophilic, density difference 130 kg/m3, density ratio 1.13, interfacial tension 2.76 mN m−1	(a) With tail at periphery, J-type planetary motion moves LP to the tail at periphery and UP to the head at centre (Fig. 4). However, the LP in the UP region cannot be depleted. With tail at Centre, the LP and UP is completely separated with the former being at head (periphery). (b) At U-O-T flow mode, stationary LP is not retained at all (Fig. 5). Similar results occur for a hydrophilic two-phase system n-butanol/acetic acid/water (4:1:5) as documented in ref. [26].	(a) The ATPS has low interfacial tension and so the solid–liquid friction force applies considerably to UP as well. With tail at periphery, whilst tangential centrifugal force pushes LP to the periphery and UP to the centre, the solid–liquid friction force drags both phases towards periphery and hence fails to completely separate the two phases. With tail at centre, UP is subject to the 2 forces towards centre whereas LP to the 2 forces in the opposite directions. Provided tangential centrifugal force can overcome the solid–liquid friction force, the two phases can then be completely separated. This situation indicates that the ATPS has a sufficiently high density difference. In this vein, one can explain all the results shown in Fig. 4. (b) As shown in Fig. 6(v) and Section 3.3, UP coming from periphery moves quickly to centre due apparently to tangential centrifugal force and against the solid–liquid friction force. Because of the solid–liquid friction force, UP accumulated near the central terminal cannot be expelled and so LP is instead repelled slowly with the upper mobile phase. Eventually, the entire LP in the column is gradually replaced by the UP. Indeed, similar experimental results were reported earlier in refs. [13,22–26]. This outcome results from striking a fine balance for retaining both LP and UP. For n-butanol/acetic acid/water (4:1:5) phase system, addition of NaCl into the phase forming water has reduced the settling time from 38.5 s to 26.5 s [26,27].

^aFurther examples and the associated phase physical properties are given in refs. [12,18,26,27].