Sven H. Behrens and David G. Grier
Dept. of Physics, James Franck Institute, and Institute for Biophysical Dynamics
The University of Chicago, Chicago, IL 60637
Date: May 11, 2001
One of the most prominent open questions in colloid physics concerns the influence of spatial confinement on the interaction between charged colloidal particles. For an isolated pair of similar particles in an electrolyte solution, the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory  predicts a short-ranged van der Waals interaction and an electrostatic repulsion, which, for well-separated spherical macroions, takes the familiar screened Coulomb form
Experimentally, the pair interaction energy can be assessed by analyzing the relaxation of two particles released from optical traps [3,4], or by interpreting the liquid structure of an equilibrated dispersion [5,6,7]. Both techniques consistently reveal that polystyrene spheres dispersed in deionized water and closely confined between parallel glass walls experience a long-range pair attraction [6,7,8,9] qualitatively inconsistent with DLVO predictions, and more generally with mean-field  or local density theories . Unconfined particles, on the other hand, repel each other as expected [3,5,8],
This Letter reports measurements of colloidal pair interactions in equilibrium near a single wall. The only previous one-wall measurement used optical tweezers to position highly charged polystyrene spheres near a charged glass surface, and then released the spheres to measure their interactions . Although this measurement revealed attractions consistent with those observed between two walls, Squires and Brenner  showed that they could have resulted from hydrodynamic flows excited by the spheres' retreat from the wall, a purely kinematic effect. The resulting hydrodynamic interaction would mask any other attraction in non-equilibrium measurements near a single wall. It cannot have influenced the equilibrium long-ranged attractions deduced from the structure of colloidal monolayers confined between two walls [6,7]. Nor does it account for the anomalous attractions measured with optical tweezers between two walls; reanalyzing the data from Ref.  reveals no sign of hydrodynamic memory. Moreover, indirect evidence based on the structure and dynamics of metastable crystals [12,14] suggests that a single wall can induce like-charge attractions in suspensions of sufficiently low ionic strength. Our measurements therefore focus on this regime.
Because of their high specific mass ( ), the particles sediment to the bottom wall in a matter of minutes, reaching an equilibrium height where the force due to gravity is balanced by the spheres' electrostatic interaction with the wall's surface charge. The spheres' unvarying appearance under the microscope confirms their out-of-plane excursions to be smaller than . Limited out-of-plane motion reduces the possibility of projection errors, which have been identified  as a concern in earlier studies.
The particles were observed with an inverted optical microscope (Olympus IMT-2), using a N.A. 1.4 oil immersion objective and a video eyepiece. These image an area in the pixel field of view of an attached charge-coupled device camera. The particles' motions were recorded at 30 frames per second before being digitized with a (MuTech MV-1350) frame grabber. Standard image analysis techniques  were used to locate the spheres to within 30 nm.
Following the principle adopted in earlier studies, we measure the spheres' pair potential by compiling histograms of equilibrium pair separations into the two-dimensional pair correlation function, . In the limit of infinite dilution, is related to the pair interaction energy through the Boltzmann distribution, , where is the areal density of spheres. For finite concentration, on the other hand, the radial distribution function also reflects neighboring particles' influence, and generally
Although straightforward in principle, imaging measurements of are subject to subtle sources of error which can introduce spurious features into . These errors arise principally from three sources: truncation by the limited field of view, statistical undersampling of suspensions' slow dynamics, and uncorrected many-body correlations. Previous studies addressed some, but not all, aspects of these errors [5,6,7]. Consistency among their results reinforces their qualitative conclusions, while leaving some doubt regarding their quantitative accuracy. For this reason, we outline our methods.
The pair correlation function measures the mean number of particles per unit area separated from any given sphere by displacement . This average is calculated in practice by counting the number of -pairs in a recorded image and normalizing by the number of particles actually tested for a partner at separation , i.e. by the number of particles in the ``overlap area'' , where is the set of points in the field of view and is the same set displaced by . Summing over angles yields and hence .
The field of view in Fig. 1 contains too few particles to sample accurately. Increasing the field of view to improve statistics would degrade particle tracking performance . Consequently, results from independent images must be averaged to improve statistics. Unless the particles are given time to redistribute between snapshots, however, these additional images will not shed light on the suspension's equilibrium properties so much as improve the sampling of a particular transient distribution. The need for statistically independent samples implies that the period over which is averaged must be chosen with care.
If the monitored area were so large that edge effects could be neglected, then a histogram of the particle separations with bin width would give as the average number of particles separated by distance from any given particle, normalized by the corresponding value for an ideal system of noninteracting particles. Given pairs at separation in a snapshot,
Because the experimental duration increases so rapidly with dilution and because controlling concentration, temperature, and ionic strength over long periods can be difficult, statistical accuracy strongly favors larger particle concentrations. On the other hand, extracting the pair potential from measured correlations becomes increasingly difficult as increases. The areal densities between and 0.1 chosen for this study require no more than 30 minutes sampling and represent a compromise between statistical and interpretive accuracy.
Our experimental results for and at different concentrations are shown in Fig. 2. The curves indicate a repulsive core interaction causing particle depletion from a zone about wide. Beyond this is a preferred nearest-neighbor separation between and and the oscillatory correlations typical of a structured fluid. The depth of the minimum in clearly depends on and so reflects at least some many-body contributions.
To ensure that none of the observed correlations result from inhomogeneities in the glass surface's properties, we compared two-dimensional histograms of recorded particle positions with analogous histograms for uniformly distributed random data sets. Differences in these histograms' first two moments vanish with increasing delay time, confirming that each particle's position becomes uncorrelated over time as expected. Thus the substrate potential appears to be featureless on the length- and timescales of our experiment, to within our resolution.
Provided that is free from experimental artifacts, reliable approximations for can be obtained from using the Ornstein-Zernike integral equation with appropriate closure relations. Good results for ``soft'' potentials are typically achieved with the hypernetted chain (HNC) approximation, whereas the Percus-Yevick (PY) approximation is known to be a better choice for hard spheres. The pair interaction potential can be evaluated numerically in these approximations as
The absence of minima in the pair potential confirms that
crowding induces the observed oscillations in ,
while the underlying interaction is purely repulsive.
The solid curves in Fig. 3 are fits
to Eq. (1) for , , and
an arbitrary additive offset.
The fitting parameters
listed in Table 1
are consistent with surface charging due to silanol dissociation
at an ionic strength around
The DLVO theory's success at characterizing our data might seem surprising given the experimental geometry. However, the DLVO form has been shown to capture the leading-order behavior in the Poisson-Boltzmann approximation, with the bounding wall introducing only an additional dipole repulsion [19,20] to lowest order. However, the dipole correction is predicted  to be weaker than our experimental resolution over the experimentally accessible range of interparticle separations. However, long-range attractions of the previously reported strength [6,7,8,12] would have been resolved.
The decay of provides an estimate  of for the sample with . The measurement period therefore yields with a relative error given our binning resolution . The other samples yield comparably good results.
The absence of a measurable inter-particle attraction in this experiment should be interpreted with care. Our silica spheres carry lower charge densities than the polystyrene particles used in earlier studies, and the sphere-wall separation is considerably smaller. A wall-induced equilibrium attraction weaker than our experimental resolution would have to depend strongly on these parameters. The measured structure of our colloidal monolayers suggests instead that like-charged particles repel each other in pure water, even near a charged wall. Our observations support the electrohydrodynamic explanation  for the attraction measured with optical tweezers in Ref. , thus distinguishing that attraction from those seen in dispersions confined between two walls, and those implicated in observed anomalous phase behavior in bulk suspensions.
We are grateful to Todd Squires, Michael Brenner and Vladimir Lobaskin for enlightening conversations. This work was supported by the National Science Foundation through Grant Number DMR-9730189 and by the Swiss National Science Foundation.