Total internal reflection microscopy is used to measure the total potential energy of interaction between a 6 μm polystyrene (PS) latex bead and either a bare glass microscope slide or a glass slide spin-coated with a 1 μm thick PS film, when the two interacting bodies are separated by 10−300 nm of aqueous solution having an ionic strength between 0.5 and 3 mM. In particular, these are the first measurements of van der Waals interaction between microscopic bodies of PS across water, for which the dielectric spectra are well-known. Under these conditions the bead is levitated above the slide by double-layer repulsion. After the gravitational contribution is subtracted, the potential energy profile displays a minimum of 0.5−2.3kT formed by long-range van der Waals attraction and shorter-range double-layer repulsion. The attraction was detectable at distances up to 200 nm. At separation distances greater than 100 nm (energy < 0.5kT), the measurements agree well with predictions using Lifshitz theory to predict the interaction of two PS half spaces, coupled with Derjaguin's approximation to account for the curvature of the sphere. At all separations, both retardation and screening are very important to the van der Waals interaction. As the separation becomes smaller than 100 nm, the measured interaction becomes weaker than predicted. Using atomic force microscopy (AFM), we observed asperities with heights up to 10 nm on the spin-coated PS film and up to 30 nm on the latex bead. If our experimental “zero” separation corresponds to contact of the largest asperities, the separation distance used in the theory should be larger than that measured. Shifting the theoretical curve by the sum of the asperity heights causes the theory to shift from overpredicting the van der Waals attraction to underpredicting it. We suggest a new theory in which roughness is treated as a diffuse film whose composition varies from pure PS at the inner surface to pure water at the outer surface. The composition profile can be determined independently from the histogram of elevations measured with AFM.

This work involves the development of a novel technique that integrates total internal reflection and video microscopy methods to simultaneously measure single particle and ensemble average particle-surface interactions. For the 2 mum silica colloids and glass coverslip used in this study, particle size polydispersity is found to be a dominant factor in determining the distribution of single particle profiles about ensemble average profiles. In conjunction with this observation, chemical and physical nonuniformity are not evident in any of our measurements even with sensitivity to interactions on the order of kT. One advantage of using ensemble averaging in conjunction with time averaging is the ability to dramatically decrease the time required to measure average particle-wall interactions which scales inversely with interfacial particle concentration. A number of experimental issues are addressed in the development of this technique including (1) combining single particle distribution functions, (2) statistical sampling of distribution functions using both time and ensemble averaging, and (3) correcting overlapping scattering signals between adjacent particles. The capabilities of the ensemble averaging technique are also demonstrated to provide unique measurements of particle-surface interactions in metastable systems by selecting only height excursions of levitated particles when calculating potentials. Ultimately, this new technique provides several important advantages over single particle measurements, which provides a foundation for measuring interactions in increasingly complex interfacial systems.

Total internal reflection microscopy is a technique for monitoring changes in the distance between a single microscopic sphere and a flat plate by measuring the intensity of light scattered by the sphere when illuminated by an evanescent wave. A histogram of scattering intensities can be used to construct the potential energy profile as a function of distance relative to the most probable distance. Thus potential energies can be measured to within a fraction of kT while changes in distance can be measured to within 1 nm. An autocorrelation of the scattering intensities can be used to deduce an average diffusion coefficient of the sphere, which is found to be only a few percent of the Stokes–Einstein value, owing to the close proximity of the plate. The analysis of the intensity-autocorrelation function presented here can be used to deduce an absolute value for the most probable separation distance, without a priori knowledge of the functional form of the PE profile and in the presence of a constant background scattering intensity. This “hydrodynamic” separation distance is found to be within a few percent of the “optical” separation distance found independently by comparing the intensity at the most probable distance with the intensity of the same particle in contact with the plate. Since the particle does not need to be brought into contact with the plate, the hydrodynamic method is well suited for determining the absolute separation distance with deformable particles like liquid droplets, vesicles or biological cells. Moreover, the hydrodynamic separation can be immediately calculated without any additional experiments. However, accurate determination of the hydrodynamic separation requires an accurate value for the particle size, which must be determined independently.

The precision and accuracy of measurements of the diameter and electrophoretic mobility (mu) of polymeric nanoparticles is compared using four different analytical approaches: carbon-nanotube-based Coulter counting, dynamic light scattering (DLS), transmission electron microscopy (TEM), and phase analysis light scattering (PALS). Carbon-nanotube-based Coulter counters (CNCCs) use a 132 nm diameter channel to simultaneously determine the diameter (28-90 nm) and mu value for individual nanoparticles. These measurements are made without calibration of the CNCC and without labeling the sample. Moreover, because CNCCs measure the properties of individual particles, they provide true averages and polydispersities that are not convoluted into the intrinsic instrumental response function of the CNCC. CNCCs can be used to measure the size of individual nanoparticles dispersed in aqueous solutions, which contrasts with the TEM-measured size of individual dehydrated particles and the ensemble size averages of dispersed particles provided by DLS. CNCCs provide more precise values of mu than PALS.

Colloidal matter with a wide range of materials, sizes, and configurations was built with opto-thermophoretic assembly.