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Drop-on-demand inkjet technology offers a method through which nano- to microgram quantities of a substance can be delivered to a precise target location. However, few measurement techniques exist to quantify the material dispensed in quantities representative of those deposited on a typical target. While many methods exist that are suited to quantitate the cumulative material dispensed by large numbers of drops, few are capable of adequately quantifying subthousand drop counts. This gap in the quantitation methodology is significant since typically the first few drops in a burst have different mass from that of the subsequent drops. A UV–visible spectroscopic method is presented here as a means to quantify the dispensed material associated with drops numbering in the few tens to hundreds, with this method exhibiting excellent repeatability and reproducibility without the need for lengthy calibrations or sample preparation times. The resolution and precision of this method make it an attractive choice for...

A finite difference time domain (FDTD) algorithm with nonstandard finite differences is used to compute light scattering by colloidal particles. The objective of this research is to simulate scattering for cases where multiparticle scattering is significant and where particle shape is arbitrary and not limited to regular geometries commonly illustrated in mathematical solutions such as the Mie theory. In this paper scattering characteristics computed using the FDTD method are compared with known analytical solutions for a number of cases for which a two-dimensional treatment is sufficient. These include scattering by a single infinite cylinder, a hollow cylinder, and two parallel cylinders. The numerical results were found to agree well with the analytical solutions. Results are also given for cases where multiparticle scattering is significant and analytical solutions are extremely difficult if not impossible to obtain.

Experimental results indicate that the electric field governing retention in the electrical field-flow fractionation (FFF) experiment is very sensitive to experimental parameters such as the flow rate and carrier conductivity. In this paper, the first of a two-part study on the mechanics of electrical FFF, we present an electrical model of the channel and characterize the dependence of many of the system parameters. Understanding the nature of the electric field in the channel is essential for using electrical FFF as a separation tool as well as a physical characterization technique where electrophoretic mobility and particle size are estimated.

Experimental work presented in this paper demonstrates that retention in electrical field-flow fractionation shows a very strong dependence on the amount of injected sample, even for very low concentrations. This concentration dependence on retention is shown to result from conductivity differences between the colloidal particles and the carrier fluid. As a result of this concentration dependence, the electric field is a function of position inside the channel. This fact is important when electrical FFF is used for determining electrophoretic mobilities and particle sizes because the analyst must be careful in interpreting the physical origin of retention changes. Furthermore, it is shown through experiment that residual salt in the injected sample buffer can affect retention. A transport model is developed that characterizes the relaxed zone profile; computation of these developed equations shows the extent of deviation from the standard transverse exponential concentration profile.

The combined effects of hindered diffusion and adsorption in a single pore can be used as the basis for modeling diffusion and adsorption of macromolecules such as proteins in porous media. We examine here the influence of pore tapering, which affects both the hindrance and adsorption effects on the overall transport rate. The governing equation is developed by averaging the continuum diffusion equation across the pore, and it is solved numerically for the case of a cylindrical pore with radius decreasing linearly with axial position. The effect of the taper on axial concentration profiles is to give rise to a pronounced inflection relative to the shape seen for a straight pore, and the net uptake rate is altered as well. These results indicate that extraction of pore structure details from solute uptake information is problematic, and that empirically correlated pseudocontinuum transport models may be more reliable.

The potential energy as a function of distance is obtained for the double-layer interaction of two spherical colloidal particles. The Poisson-Boltzmann equation is solved numerically, using a collocation technique, for two spheres having various sizes and for constant potential and constant charge boundary conditions. The potential distribution is determined and then used to find the change in the electrical energy, the entropy, and the chemical part of the free energy as the two particles are brought into contact from infinity. Various approximate methods used previously, depending on surface conditions and geometry, are found to work quite well for the constant potential case, but for the constant charge case, they are not adequate for most of the parameter space. The interaction energy profiles are found to be functions of the curvature of the interacting spheres, and the dependence is quite strong for the constant charge case, in which the relative magnitudes of the electrical energy and entropy change significantly as the radii of the interacting spheres are changed for a fixed Debye length.

The coupling of field-flow fractionation (FFF) and multiangle light scattering (MAIS) detectors is complementary in that the MALS system allows particle characterization when a narrow dispersity particle population is present in the detector. The fractionation process provides this narrow dispersity. Utilizing discrete particle simulations of FFF and optical calculations based on both the Mie theory of particle scattering and Rayleigh-Gans-Debye (RGD) scattering theory, the extent of polydispersity that can be tolerated for accurate particle quantitation is explored. It is found that flow, electrical, and sedimentation FFF provide adequate separation for accurate particle quantitation by MALS. The Mie theory is more accurate than the RGD theory, which is known to deviate at higher particle size. Low error in the measurement of mean diameters is found when only the particle diameter is of interest. It is shown that the reconstruction of the particle size distribution from time slice data is distorted due to errors in concentration, which result from finite polydispersity and other effects. A number of procedures are evaluated in restoring the size distribution to higher accuracy. None of these procedures is deemed of general purpose and none of these is reliable. The best results are obtained when fractionation is conducted under the minimal possible outlet polydispersity and when steric effects are minimized. In addition, best results are had for inherently narrow dispersity colloidal materials.

As inkjet printing technology is increasingly applied in a broader array of applications, careful characterization of its method of use is critical due to its inherent sensitivity. A common operational mode in inkjet technology known as drop-on-demand ejection is used as a way to deliver a controlled quantity of material to a precise location on a target. This method of operation allows ejection of individual or a sequence (burst) of drops based on a timed trigger event. This work presents an examination of sequences of drops as they are ejected, indicating a number of phenomena that must be considered when designing a drop-on-demand inkjet system. These phenomena appear to be driven by differences between the first ejected drop in a burst and those that follow it and result in a break-down of the linear relationship expected between driving amplitude and drop mass. This first drop, as quantified by high-speed videography and subsequent image analysis, can be different in morphology, trajectory, velocity, and volume from subsequent drops within a burst. These findings were confirmed orthogonally by both volume and mass measurement techniques which allowed quantitation down to single drops.