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Particle displacement in aqueous suspension arising from incident radiant energy Kevin W Kimura, and Gerald H. Pollack Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5048535 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015
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Langmuir
Particle displacement in aqueous suspension arising from incident radiant energy
Kevin W. Kimura and Gerald H. Pollack Department of Chemical Engineering, Department of Bioengineering, University of Washington, Seattle Washington 98195, United States
ABSTRACT Colloidal particles in aqueous suspension generally sediment uniformly. By contrast, we found that suspensions of latex microspheres in polystyrene Petri dishes deviated sharply from the expected pattern when various objects were positioned immediately outside those dishes. When small coin-like metal discs were positioned immediately beneath the Petri dish, the microspheres sedimented to a point just above those discs. Other materials, including glass and wood, produced similar results, though less pronounced. After the microspheres had sedimented, shifting the metal to another position beneath the dish caused the microspheres to follow. Various control experiments ruled out trivial explanations. In concordance with earlier results, it appears that the infrared energy generated by the various materials draws microspheres, resulting in the unusual sedimentation patterns. The results have significant implications for the mechanism of sedimentation, particularly for the role of charge in that process.
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INTRODUCTION Aqueous colloidal suspensions are of vital importance because of their ubiquitous presence throughout nature1. Colloid sedimentation is thus a heavily researched subject2,3. Understanding sedimentation is of interest in diverse realms, including wastewater treatment, industrial settings, environmental protection, and biological organisms4. Sedimentation is also used to characterize colloids, as embodied in techniques such as field-flow fractionation5,6. Modeling has helped to achieve further understanding of this phenomenon7. Extensive research notwithstanding, numerous properties of colloids and water remain under investigation. This includes the interfacial zone surrounding each suspended colloidal particle and, in particular, the influence of radiant energy on the interfacial zones8. Recent work sheds fresh light on both colloidal dynamics and the role of the interfacial water that surrounds each colloidal particle. Layers of interfacial water, called “exclusion zones” or “EZs” because of their profound exclusionary nature, build next to hydrophilic surfaces. Interfaces of microspheres in aqueous suspension have also been shown to develop EZs. Hence, these interfacial zones may influence colloidal dynamics, including aggregation and sedimentation. While investigating the impact of variables such as light and charge on microsphere interactions, we came upon an unexpected phenomenon: attraction of suspended microspheres to objects placed outside the chamber. Instead of settling uniformly over the bottom of the chamber, the microspheres accumulated directly over various objects placed immediately beneath. Experiments were performed to explore this phenomenon and deduce its origin. MATERIALS AND METHODS
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The main material used in this experiment was a dilute suspension of carboxylate microspheres (Polysciences, Inc., Polybead® carboxylate microspheres, 1 µm, 08226-15). Five drops of microsphere concentrate were added to 50 mL of deionized water, obtained from a Barnstead NANOpure Diamond filter. This produced a concentration of ~1.14x107particles/L. Five mL of this suspension was placed in a Fisherbrand polystyrene Petri dish (50 mm diameter x 15 mm height) and covered with a plastic lid. Use of the lid was important for minimizing the disruptive effects of evaporation and ambient air currents. By approximately 24 hours, the microspheres uniformly settle to the bottom of the Petri dish, covering the entire floor, as shown in Figure 1.
Figure 1. Control experiment. (A) Initial microsphere suspension. (B) Sedimented microsperes after 24 hours. Lid was removed immediately prior to taking these pictures. Microspheres sediment moderately uniformly in the center. All pictures were taken with a Kodak EasyShare P880 on the “close-image” setting, 20 cm from the Petri dish under the ambient light. For obtaining IR images, we used an uncooled VOx microbolometer (spectral response: 7.5-14 microns; sensitivity:
