Joshua A. Dijksman

Improved piercing of microneedle arrays in dermatomed human skin by an impact insertion method

An electrical applicator was designed, which can pierce short microneedles into the skin with a predefined velocity. Three different shapes of microneedles were used, namely 300 mum assembled hollow metal microneedle arrays, 300 mum solid metal microneedle arrays and 245 mum hollow silicon microneedle arrays. The latter are available as 4x4, 6x6 and 9x9 arrays. When using a velocity of 1 or 3 m/s reproducible piercing of dermatomed and full thickness human skin was evident from the appearance of blue spots on the dermal side of the skin after Trypan Blue treatment and the presence of fluorescently labeled particles in dermatomed skin. Manual piercing did not result in the appearance of blue spots. Transport studies revealed that i) piercing of microneedles with a predefined velocity into human skin resulted in a drastic enhancement of the Cascade Blue (CB, Mw 538) transport, ii) A higher piercing velocity resulted in a higher CB transport rate, iii) The CB transport rate was also dependent on the shape of the microneedles and iv) no difference in transport rate was observed between 4x4, 6x6 and 9x9 hollow silicon microneedle arrays.

Invited Article: Refractive index matched scanning of dense granular materials

We review an experimental method that allows to probe the time-dependent structure of fully three-dimensional densely packed granular materials and suspensions by means of particle recognition. The method relies on submersing a granular medium in a refractive index matched fluid. This makes the resulting suspension transparent. The granular medium is then visualized by exciting, layer by layer, the fluorescent dye in the fluid phase. We collect references and unreported experimental know-how to provide a solid background for future development of the technique, both for new and experienced users.

Jamming, Yielding and Rheology of Weakly Vibrated Granular Media

We establish that the rheological curve of dry granular media is nonmonotonic, both in the presence and absence of external mechanical agitations. In the presence of weak vibrations, the nonmonotonic flow curves govern a hysteretic transition between slow but steady and fast, inertial flows. In the absence of vibrations, the nonmonotonic flow curve governs the yielding behavior of granular media. Finally, we show that nonmonotonic flow curves can be seen in at least two different flow geometries and for several granular materials.

Spanning the scales of granular materials through microscopic force imaging

If you walk on sand, it supports your weight. How do the disordered forces between particles in sand organize, to keep you from sinking? This simple question is surprisingly difficult to answer experimentally: measuring forces in three dimensions, between deeply buried grains, is challenging. Here we describe experiments in which we have succeeded in measuring forces inside a granular packing subject to controlled deformations. We connect the measured micro-scale forces to the macro-scale packing force response with an averaging, mean field calculation. This calculation explains how the combination of packing structure and contact deformations produce the observed nontrivial mechanical response of the packing, revealing a surprising microscopic particle deformation enhancement mechanism.

Granular flows in split-bottom geometries

There is a simple and general experimental protocol to generate slow granular flows that exhibit wide shear zones, qualitatively different from the narrow shear bands that are usually observed in granular materials. The essence is to drive the granular medium not from the sidewalls, but to split the bottom of the container that supports the grains in two parts and slide these parts past each other. Here we review the main features of granular flows in such split-bottom geometries.

Rheology of weakly vibrated granular media.

We probe the rheology of weakly vibrated granular flows as function of flow rate, vibration strength, and pressure by performing experiments in a vertically vibrated split-bottom shear cell. For slow flows, we establish the existence of a vibration-dominated granular flow regime, where the driving stresses smoothly vanish as the driving rate is diminished. We distinguish three qualitatively different vibration-dominated rheologies, most strikingly a regime where the shear stresses no longer are proportional to the pressure.

The role of tap duration for the steady-state density of vibrated granular media

We revisit the problem of compaction of a column of granular matter exposed to discrete taps. We accurately control the vertical motion of the column, which allows us to vary the duration T and the amplitude A of single-cycle sinusoidal taps independently. We find that the density of the material at the reversible branch depends both on A and T. By comparing the densities on the reversible branches obtained for a range of values of T, we find that we can collapse all data when plotted as a function of A/T, which scales similar to both the liftoff velocity and the time of flight of the packing. We further show that switching between states obtained for different A and T, but chosen such that their densities on the reversible branches match, does not lead to appreciable hysteresis. We conclude that the appropriate control parameter for sinusoidal tapping is not the peak acceleration Γ~A/T2, as is usually assumed, but rather ΓT~A/T.

Water-repairable zwitterionic polymer coatings for anti-biofouling surfaces

We developed a strategy to prepare new types of zwitterionic polymer network (ZPN) coatings that display excellent self-healing and anti-biofouling properties. Upon mechanical damage (scratching), the coatings can be easily repaired within 1 min by immersion in water, and this self-healing property can even be achieved for nanoscale-thick coatings. Both the mechanical properties and the anti-biofouling characteristics are repaired after healing.

Particle diffusion in slow granular bulk flows

We probe the diffusive motion of particles in slowly sheared three-dimensional granular suspensions. For sufficiently large strains, the particle dynamics exhibits diffusive Gaussian statistics, with the diffusivity proportional to the local strain rate --consistent with a local, quasistatic picture. Surprisingly, the diffusivity is also inversely proportional to the depth of the particles below the granular surface --at their free surface, the diffusivity thus appears to diverge and is ill defined. We find that the crossover to Gaussian displacement statistics is governed by the same depth dependence, evidencing a non-trivial strain scale in three-dimensional granular flows.

Spore movement driven by the spore wall in an eusporangiate fern

Abstract We report the ejection of spores on sporangium dehiscence in the eusporangiate fern genus Angiopteris (Marrattiaceae, Marattiales). Using normal and high-speed video we document movement of the spores and using light and electron microscopy we study the structural changes associated with the movement. The sudden and spontaneous movement covering distances of up to several mm cannot be ascribed to action of the sporangium. We find that cavitation between exospore and perispore wall is the most likely responsible mechanism. We suggest that spore ejection by movement of a spore wall layer may have driven the evolution of an elaborate multilayered spore wall in ferns.