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Conclusions

Does geometric confinement induce equilibrium attractions between like-charged colloidal spheres? At first glance, the observation of purely repulsive interactions in Sec. 5 suggests that attractions measured with optical tweezers in Sec. 2.4 should be interpreted as a kinematic effect along the lines proposed by the Squires-Brenner theory in Sec. 4. However, the silica spheres discussed in Sec. 5 carry a much lower effective charge than the polystyrene spheres for which confinement-induced attractions have been observed. If a putative wall-induced attraction depends at least linearly on the spheres' surface charge density, then it might not have been observable at the energy resolution of our equilibrium interaction measurements. Furthermore, the silica spheres' interactions were measured near one wall, rather than between two. The degree of confinement may make a quantitative, or even qualitative difference to the nature of the effective pair interaction.

Are reports of equilibrium like-charge attractions to be trusted? Deliberately undersampling the data used to compute $ g(r)$ in Sec. 5 does indeed lead to the appearance of long-ranged attractions, but only in rare instances, and not systematically. While we cannot yet rule out experimental error as the origin of the attractions observed in Refs. [23] and [24], the possibility appears remote. Further measurements are in progress to isolate the role of confinement, surface charge, and screening length on the nature of colloidal electrostatic interactions in equilibrium.

For the time being, we are left with the tentative conclusion that geometric confinement can induce like-charge attractions in charge-stabilized colloid. Such an attraction appears to be outside the realm of local density theories.

If like-charge colloidal attractions are an equilibrium phenomenon, it cannot result from electrohydrodynamic coupling. Even so, the Squires-Brenner theory opens a new chapter in our understanding of electrokinetic effects in charge stabilized suspensions. Certainly, it sheds new light on the compression mechanism exploited in Sec. 2.3 to create metastable superheated crystals. Future advances along these lines will inspire further exploration into the boundless variety of complex, beautiful and useful properties engendered by the interplay of electrostatic and hydrodynamic interactions in colloidal suspensions.


next up previous
Next: Acknowledgments Up: Interactions in Colloidal Suspensions: Previous: Electrostatics Near One Wall
David G. Grier 2001-01-16