Initial experiments followed earlier studies8–10
showing a negatively charged EZ. A lingering question was whether the negative charges of the EZ might be balanced by complementary positive charges beyond the EZ, namely by protons or hydronium ions.
To explore this possibility, the time course of pH change in the vicinity of Nafion was measured. shows that as the EZ was built, the pH in the zone beyond went sharply downward; it then recovered to a value lower than the initial value. Farther from the Nafion surface, the phasic downturn began later. The results implied a wave of protons emerging from the direction of the EZ, leaving the water at lower pH than prior to the time the EZ had begun building.
Figure 1 (A) Time course of pH change following addition of water to Nafion sheet. Values of pH were measured at 5 s intervals using a miniature pH probe positioned at three distances from the Nafion surface, as indicated in the legend. A wave of protons is generated (more ...)
To confirm and extend these pH measurements to longer distances, pH-sensitive dyes were added, and the results are shown in . The clear zone just above the Nafion surface implies that the pH-sensitive dyes are excluded from the EZ. Beyond the EZ, the color is red-orange, indicating a pH less than 3. Farther from the Nafion, the pH was lower than the Nafion-absent control, and eventually, at 10 mm from the Nafion surface and beyond, pH values were similar to controls. Hence, the results imply that the zone beyond the EZ is indeed populated by abundant protons that appear to be associated with EZ buildup.
A clue for the source of energy for EZ buildup came after having inadvertently left the experimental chamber on the microscope stage overnight. The EZ size had diminished overnight; but after turning on the microscope lamp to full intensity, the EZ size began to increase, restoring itself within minutes to its former size, ~300 µm. With preliminary evidence that light could expand the EZ, we investigated systematically whether the energetic source for EZ buildup might indeed be radiant energy.
Liquid water absorbs strongly at wavelengths of 2.9–3.25 µm, which corresponds to the fundamental O–H stretching mode.11–13
In this spectral range, the most accessible commercially available source was an LED radiating at 3.1 µm with full width at half-maximum (fwhm) of 0.55 µm; hence, the first light source used was LED31-PR.
Nafion tubing was suffused with a 1-µm carboxylate-microsphere suspension with a 1:500 volume fraction, to a depth of ~1 mm. The chamber was made using a thin cover glass stuck to the bottom of a 1-mm thick cover slide with a 9-mm circular hole cut in the center and was placed on the stage of the microscope. A pinhole was used to obtain an incident beam of restricted diameter. A fabricated holder integrated the pinhole and LED into a single unit with the LED mounted close to the pinhole. The LED-pinhole axis was vertically oriented.
The baseline EZ size was first established before measuring IR-induced EZ expansion. The sample was prepared and initially left in the dark. Once enough time had passed for the EZ to stabilize, approximately 5 min, the microscope lamp was turned on briefly to take photomicrographs showing the baseline EZ size. The EZ size after IR irradiation was compared to this baseline size to compute the expansion ratio in each run.
To minimize any effects of microscope illumination on EZ size, the microscope lamp was turned on only when necessary for visualizing the EZ, and then immediately turned off. A green filter with a sharp peak at 550 nm was used to further minimize incident radiation. Immediately following the baseline measurements, the incident IR source was turned on. Optical power output was 33 µW, and the estimated power incident on the sample through the pinhole was ~2.4 nW. After 5 min of exposure, the IR LED assembly was removed and the EZ was immediately photographed through the microscope. From the representative records shown in , it is apparent that even with modest IR exposure, the EZ grew to approximately three times its control size.
Aware of the potential for contamination by even brief microscope-light illumination, appropriate controls were carried out. The sample was left for 5 min with and without the microscope light turned on. The intensity of incident light was kept the same as in all other experiments, including the green filter. The EZ size was 280 ± 24.1 µm with light and 260 ± 13.3 µm without (n = 5). Hence, even with microscope illumination of a far longer duration than in actual experiments (5 min vs several seconds), the effect on EZ size was modest. Apparently, the extensive expansion effects observed were due solely to the incident IR radiation.
We also tracked the EZ width’s time course. This was carried out not only with the 3.1-µm source, but also with the 2.0-µm and 1.75-µm sources (fwhm = 0.16 and 0.18 µm, respectively). For the latter two sources, intensities were maintained at approximately 190 µW; but for the 3.1-µm source, power was kept at the maximally attainable value, 33 µW.
During a 10-min exposure at all three wavelengths, EZs continued to expand approximately linearly (). The largest effect was seen at 3.1 µm, despite lower incident power. To determine whether the EZ continues to expand beyond the 10-min exposure, the 3.1-µm source was left on at the same intensity as above for up to 1 h. The ratios increased from 3.7 ± 0.10 (10 min) to 4.7 ± 0.12 (30 min) and to 6.1 ± 0.17 (1 h), respectively. Hence, the EZ continues to expand with continued exposure for up to at least one hour. Longer durations were deemed unreliable, as evaporation became noticeable; hence measurements were suspended.
(A) EZ expansion as a function of exposure time, for three IR sources (lower power for LED31-PR). (B) EZ expansion ratios as a function of time during 10 min exposure at different intensities using LED20-PR.
Postillumination EZ-size dynamics were examined as well. When the infrared light was turned off after 5 min of exposure, the EZ width remained roughly constant with fluctuations for about 30 min; then, it began decreasing noticeably, reaching halfway to baseline levels in typically ~15 min.
To determine the effect of beam intensity on EZ expansion, the 2-µm source was employed at three power levels, 0.21, 0.34, and 1.20 mW. The rate of EZ expansion increased with an increase of incident power ().
The results of show that, at a given wavelength, EZ expansion is a function of both time and intensity. Hence, EZ growth depends on the cumulative amount of incident energy induced charge separation.
To test whether the expansion might arise out of some unanticipated interaction between the incident radiation and the particular type of microsphere probe, microspheres of different size and composition were tested. For carboxylate microspheres of diameters 0.5, 1, 2, and 4.5 µm at the same volume concentrations (1:500), mean expansion ratios for 5 min of exposure of 3.1-µm radiation were the following: 2.41, 2.97, 3.08, and 3.34, respectively (n = 6). For 1-µm microspheres made of carboxylate, sulfate, and silica under conditions the same as above, expansion ratios were 2.97, 3.10, and 1.50. Hence, some material-based and size-based variations are noted–the latter arising possibly because of different numbers of particles per unit volume–but appreciable radiation-induced expansion was nevertheless seen under all circumstances and with all materials. Hence, the existence of the light-induced expansion effect is not material-specific.
We also explored the effect of IR illumination at different depths relative to the water/air interface. Interestingly, EZ expansion was observed well below the water surface. This result is unexpected given the previously reported short penetration depth of IR in water.14,15
Nevertheless, IR effects extending far beyond the expected penetration depth have been reported and attributed to coherent energy redistribution of IR-induced excitation of surface layers via pressure waves.16,17
A more systematic time-correlated approach will be needed to determine whether any such mechanism might apply here.
Controls for Temperature
Infrared absorption in water causes a temperature elevation. Hence, we considered the possibility that the expansion might arise from an appreciable increase of chamber temperature. To measure local temperatures, an OMEGAETTE datalogger thermometer HH306 was used, with a stainless-steel-sheathed, compact transition ground-junction probe (TJC36 series), small enough (250 µm) to fit within the EZ. With the incident beam positioned to elicit the maximum expansion, i.e., centered 175 µm from the Nafion surface, the measured temperature increases are shown in . Radiation-induced temperature increases were modest at all positions and fairly uniform over the chamber. We also found little temperature variation with depth, implying that the thermal mass of the probe itself, immersed by varying extents for measurements at varying depths, did not introduce any serious artifact.
Temperature Increases Measured at Different Distances from the Nafion Surface after 10 min of Exposure to 3.1-µm Radiation (n = 3)
Further to this point, we recorded the dynamics of temperature rise. The temperature increase occurred steadily, reaching a plateau of ~1 °C at 10–15 min after turn-on. This plateau was attained at a time that the EZ continued to expand ( and associated text). Hence, not only was the temperature increase modest, but also the time course of temperature rise and EZ expansion were not correlated.
A principal objective was to determine EZ-expansion’s spectral sensitivity. The experimental setup was similar to that described above. The ~200-µm wide light beam emerging from the pinhole was directed to the middle of EZ, and expansion was measured 300 µm below solution surface. For the UV and visible sources, maintaining consistent optical power output at all wavelengths was achievable within ±10% by adjusting the driver current. But IR sources were considerably weaker; hence, output power was maintained at a lower level, 3 orders of magnitude lower than in the UV-visible ranges. Spectral results are therefore plotted separately.
For UV and visible ranges, all incident wavelengths brought appreciable expansion (). The degree of expansion increased with increasing wavelength, the exception being the data point at 270 nm, which was higher than the local minimum at 300 nm. The higher absorption may reflect the signature absorption peak at 270 nm characteristic of the EZ.18
Clear wavelength sensitivity was also found in the IR region, the most profound expansion occurring at 3.1 µm (). Recognizing that the optical power available for use in the IR region was 1/600 of that in the visible and UV regions, one can assume that with comparable incident power, the IR curve would shift considerably upward—continuing the upward trend evident in . Hence, the most profound effect is in the mid-IR region, particularly at 3.1 µm.
(A) Exclusion-zone expansion ratio under illumination for 5 min in the UV–vis spectral region. (B) Exclusion-zone expansion ratio under illumination for 5 min at IR wavelengths.