The initial result was the visual observation of a clear and consistent flow of water from the outside of the tube, through the hole, to the inside of the tube. This is illustrated in . The figures are representative of ten experiments each carried out identically as described earlier.
FIG. 1 (Color online) Nafion tube (a) just before and (b) just after puncture. Views are from bottom up. The wall of the Nafion tube is shown in the center of the image, with microspheres visible as dots. The suspension flows from the outside of the tube (left) (more ...)
By tracking the inward motion of microspheres through the hole, it was possible to monitor the rate of flow over time. Inward flow started out strong but dropped off to lower values after tens of minutes (see ). These lower values were sustained for several hours, with mean values of 5.7 ± 2.7 microspheres per second (n = 10 experiments). This flow persisted over a 24-hr observation time, though the rate did slowly approach zero.
Representative graph of flow rate into the tube as a function of time. (a) Ten repeated experiments showed similar characteristic decrease over time. (b) Log-log plot of same data, showing continuous decrease over time.
To check the possibility that only the microspheres, but not the microsphere suspension itself, was passing through the hole, we examined the menisci positions inside the tube as a function of time. There was a clear outward shift of both menisci starting immediately after the hole was made, indicating that the fluid, and not just the microspheres alone, flowed through the hole. Additionally, the shape of the menisci changed from concave, initially, to flat once the fluid began flowing. This change implied that it was not the menisci that were pulling, but indeed the fluid’s pressure that was pushing the menisci outward.
To check further whether capillary forces might be sucking the fluid into the tube, we eliminated the meniscus at one end, as follows. Flow was initialized and allowed to reach its near-steady rate. Once this was achieved, a syringe was used to inject additional solution into the end of the Nafion tube until the tube was completely filled so there was no meniscus. Continued observation showed no detectable change in the inward flow rate. Hence, capillary effects apparently play little role in drawing the fluid through the hole.
An additional control, to check whether the microspheres themselves might be responsible for driving the flow, was to eliminate the microspheres and use a dye. For these experiments a bolus of sodium fluorescein dye was injected outside the hole with a syringe. The dye was drawn into the tube in much the same way as the microspheres although eventual diffusion limited the general usefulness of this approach.
To test whether the underlying mechanism involved local effects only, we created a second hole ~1 cm away from the first. This was done ~1 hr after the first hole had been punched. We found that the flows were coupled; i.e., just as the second hole was punched, flow through the first hole abruptly diminished (). Meanwhile, flow through the second hole exceeded the prepuncture flow through the first hole. Both flows continued to decrease with time. This coupling implied that the flow was dependent both on local properties and characteristics of the tube system in general.
Flow as a function of time, first with a single hole and just after puncture of a second hole ~1 cm away. Representative of four experiments.
To determine whether exclusion-zone size might play a role in determining flow, we tracked inner and outer EZ sizes as a function of time, along with flow rate (). Outer EZ showed little variation with time; however, inner EZ did vary substantially over time: As inner EZ size shrank, flow diminished concomitantly. Representative data are shown in .
Flux into the tube and EZ size (a) inside and (b) outside the tube, over time. EZ size inside is tightly coupled with flow rate while EZ outside is not. Results representative of four experiments.
To test further for EZ involvement in the phenomenon, a control experiment was carried out using a Tygon tube (Cole-Parmer, Vernon Hills, IL) which exhibits no EZ. The same procedures were followed as described earlier, using a tube of similar size and diameter (i.d. 2.77 mm, wall thickness 0.86 mm). The needle produced a hole in the tube, but no flow was observed. From these observations and those of , we could draw two conclusions: First, the exclusion zone is likely to be a relevant factor for the presence of flow. And second, gravity-related hydrostatic pressure is not a factor in generating the flow, as the depths of the tubes that did and did not produce flow were the same.
In order to further explore the role of the exclusion zone in this flow, we studied whether flow dynamics might be impacted by induced changes in EZ size. Earlier research had shown the EZ to be negatively charged [1
]. Hence, by adding H+
in the form of an acid, charge neutralization could reduce EZ size; or, by adding OH−
in a base, the increased negative charge could enhance EZ size. This expectation proved accurate, and it was thus possible to test the effects of EZ size on flow rate. These tests involved creating acidic or basic suspensions, which were then substituted for the aqueous suspensions inside or outside the tube, giving four different conditions.
With a 0.01M NaOH-containing microsphere suspension introduced into the Nafion tube instead of the control suspension, the inside EZ expanded from ~0.2 to ~0.5 mm. When punctured, the inward flow was considerably greater than the control. Instead of leveling off to a rate of four to five microspheres per second, the flow leveled off at 20–25 microspheres per second (). Hence, increased inside EZ was associated with increased flow, again reinforcing the connection.
Water flow into the tube, with 0.01M NaOH solution inside tube. Representative of four experiments.
With HCl of the same concentration inside the tube, the opposite result was found—as was anticipated. The inside EZ became almost zero, compared to ~0.2 mm for the control. The flow began inward as usual, but dropped to zero by the 5-min mark. It then actually reversed direction, the outward flow increasing over the next half hour and reaching a maximum rate of 10 microspheres per second before diminishing to a slower rate (). Similar patterns were seen in each of six experiments, although the dynamics differed slightly.
Water flow into the tube, with 0.01M HCl microsphere suspension inside tube. Positive values indicate a flow into the tube. Representative of six experiments.
Acidic and basic solutions were also placed outside of the tube, rather than inside. With acid outside the tube, the flow behaved similarly to the NaOH-inside results: Inward flow was much higher than the control and remained at a higher steady rate relative to controls after 30–40 min. Significant complications were encountered when using NaOH outside the tube. Once the inward flow started, the microspheres began clumping together and precipitating out of the suspension. As a result, no significant data could be obtained for NaOH outside the tube.
For all of these four pH tests, we also tracked inside EZ size, as done with the original tests described earlier. Inside EZ consistently decreased with time, as was the case with controls.
From all of these observations we began formulating a hypothesis that could account for all of the results, which we discuss below.