On the limits of digital video microscopy

Abstract

Abstract. -We explore the limits of digital video microscopy which is established as a standard method in physics, chemistry and biology. At particle distances close to contact we observe small but systematic deviations between the optically measured and the real particle distances. This difference is caused by the overlap of the optical images between neighboring particles. Exemplarily we discuss the consequences of this effect on pair potential measurements of charge stabilized colloids in confined geometries.During recent years, the interest in experimental studies with micron-sized colloidal particles which are suspended in liquids is strongly increasing. Driven by Brownian motion, such particles rapidly sample their accessible configurational space thus making them dedicated for real time "simulations" under conditions which are not easily accessible with numerical methods [1][2][3][4][5]. Another motivation for using colloids is founded on their use as mesoscopic handles which can be attached to nanometer-sized objects. Since the position of colloidal particles can be conveniently controlled with laser optical tweezers, such tethering experiments allow to study, e.g., mechanical properties on a single molecule level [6,7]. In many of the above experiments positional information of colloidal spheres is obtained by digital video microscopy which is capable to resolve particle positions with sub pixel resolution down to about 10 nm (for a review see, e.g., [8]). However, despite the general importance and the widespread use of this technique, only few studies on the limitations of this method exist. This is particularly true for particle distances close to contact where the optical images of the colloids start to overlap. In this regime experimental investigations are mandatory because earlier calculations based on simple reflection algorithms predict a systematic overestimation of the particle distances near contact by about 80 nm [9].In this letter we investigate the accuracy to which particle distances close to contact can be extracted with digital video microscopy. Our analysis shows that due to minute optical distortions caused by overlapping particles images, systematic deviations Δr between the measured and the true particle distance occur. Since Δr changes sign as a function of the particle distance, this effect can pretend attractive components in the pair potential of entirely repulsive systems. Therefore, our findings presented in this letter may resolve the ongoing controversial debate on the apparent long-ranged attraction of likely charged colloids under confinement [10][11][12]. Letters (EPL) ; 71 (2005), 3. -S. 487-493 https://dx

Two features at the two-dimensional freezing transitions

“…The softness of the particles yields short-range repulsive pair potentials. 29 The pair potentials were directly quantified by measuring the radial distribution function, g(r ), in a dilute (i.e., area density ρ ∼10%) monolayer of spheres, [30][31][32] and the small bright-field image artifact due to the image overlap of adjacent particles 33 has been corrected by the method described in Ref. 32.…”

Re-entrant melting and freezing in a model system of charged colloids

“…14 Moreover, when the range of the interaction exceeds the mean interparticle separation, the assumption of pairwise addivity becomes questionable. 15,16 Effective attractions due to nonpairwise additivity 17,18 may help explain a range of recent experimental observations such as superheated crystals, 19 and "liquid-gas" phase separation, 20 which are not expected for purely repulsive interactions. Attractions can also be caused by correlations between small ions.…”

Like-charge attraction in confinement: myth or truth?

“…Like in previous studies on LCA, we also determined particle positions with digital video microscopy which is capable to localize particle centres with accuracy down to below 20nm. In a recent 1 paper, we demonstrated that overlapping particle images lead to small but systematic optical distortions which then result in erroneously determined particle positions 19 . If not properly taken into account, this optical artefact pretends a minimum in U(r) which exactly resembles the features previously interpreted as LCA.…”

“…We want to start our discussion by recalling that images of colloidal particles are typically much larger than their geometrical size 19 . This is well-known for transparent spherical particles and also confirmed by numerical calculations 24 .…”