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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

We report a mathematically rigorous technique which facilitates the optimization of various optical properties of electromagnetic fields in free space and including scattering interactions. The technique exploits the linearity of electromagnetic fields along with the quadratic nature of the intensity to define specific Optical Eigenmodes (OEi) that are pertinent to the interaction considered. Key applications include the optimization of the size of a focused spot, the transmission through sub-wavelength apertures, and of the optical force acting on microparticles. We verify experimentally the OEi approach by minimising the size of a focused optical field using a superposition of Bessel beams.

It is general wisdom that likely charged colloidal particles repel each other when suspended in liquids. This is in perfect agreement with mean field theories being developed more than 60 years ago. Accordingly, it was a big surprise when several groups independently reported long-ranged attractive components in the pair potential U(r) of equally charged colloids. This so-called like-charge attraction (LCA) was only observed in thin sample cells while the pair-interaction in unconfined suspensions has been experimentally confirmed to be entirely repulsive. Despite considerable experimental and theoretical efforts, LCA remains one of the most challenging mysteries in colloidal science. We experimentally reinvestigate the pair-potential U(r) of charged colloidal particles with digital video microscopy and demonstrate that optical distortions in the particle's images lead to slightly erroneous particle positions. If not properly taken into account, this artefact pretends a minimum in U(r) which was in the past misleadingly interpreted as LCA. After correcting optical distortions we obtain entirely repulsive pair interactions which show good agreement with linearized mean field theories. 82,70.Dd, A controversial debate in colloidal science has been launched in 1994 when Kepler and Fraden reported an unusual long-ranged attractive component in the pair potential of charged colloidal particles 1 . This so-called like-charge attraction (LCA) was only observed in thin cells (typical sample height H < 10µm) while the pair-interaction in bulk suspensions has been experimentally confirmed to be entirely repulsive in agreement with Poisson-Boltzmann theories 2-4 . Therefore, it was speculated that the confining plates are responsible for the deviations from theory. Soon after its initial observation LCA was also observed by other authors 5-11 which then provoked considerable theoretical interest in this phenomenon. In the meantime it has been rigorously proven that the observed attraction can not be explained within the framework of mean field theories, irrespective of whether the particles are suspended in bulk or in confinement [12][13][14] . Several other approaches beyond PoissonBoltzmann have been proposed as the origin of confinement-induced attraction. While correlation effects of the electrolyte indeed can lead to a short-ranged attraction of negatively charged colloids, none of the existing theories can account for the long-ranged attraction as observed in experiments 15,16 . Accordingly, even after more than ten years of considerable research, LCA is still one of the most challenging unsolved mysteries in colloidal science. In this study we reinvestigate the pair potential U(r) of charged colloidal particles in thin sample cells. Contrary to previous experiments where U(r) was derived from the pair correlation function g(r) in semi-dilute suspensions, here we determine the pair potential directly by measuring the probability distribution of two silica spheres. This is of particular advantage because it avoids the...

The study and application of optical vortices have gained significant prominence over the last two decades. An interesting challenge remains the determination of the azimuthal index (topological charge) ℓ of an optical vortex beam for a range of applications. We explore the diffraction of such beams from a triangular aperture and observe that the form of the resultant diffraction pattern is dependent upon both the magnitude and sign of the azimuthal index and this is valid for both monochromatic and broadband light fields. For the first time we demonstrate that this behavior is related not only to the azimuthal index but crucially the Gouy phase component of the incident beam. In particular, we explore the far field diffraction pattern for incident fields incident upon a triangular aperture possessing non-integer values of the azimuthal index ℓ. Such fields have a complex vortex structure. We are able to infer the birth of a vortex which occurs at half-integer values of ℓ and explore its evolution by observations of the diffraction pattern. These results demonstrate the extended versatility of a triangular aperture for the study of optical vortices.