Hans-Jürgen Butt

From heterogeneous to homogeneous nucleation of isotactic poly(propylene) confined to nanoporous alumina.

The crystallization of highly isotactic polypropylene confined in self-ordered nanoporous alumina is studied by differential scanning calorimetry. A transformation from a predominantly heterogeneous to predominantly homogeneous nucleation takes place if the pore diameter is smaller than 65 nm. Crystallization is suppressed with decreasing pore size, and the absence of nucleation below 20 nm pores indicates the critical nucleus size. The results reported here might enhance the understanding of nanocomposites containing semicrystalline polymers and reveal design criteria for polymeric nanofibers with tailored mechanical and optical properties.

Fabrication of microvessels and microlenses from polymers by solvent droplets

A process for the fabrication of microvessels and microlenses in polymers is presented. A drop of solvent (diameter between 15 and 150μm) is deposited by an ink-jet method onto a flat polymer substrate. After evaporation of the solvent a lenticular cavity of dimensions comparable to the former drop size is created. This cavity can be employed as a microreaction vessel, as a concave lens, or as a template for a convex lens. Diameter, depth, position, and arrangement of the microvessels on the surface can be controlled.

Visible Mie Scattering in Nonabsorbing Hollow Sphere Powders

Hollow silica nanoparticles (HSNP) with diameters comparable to visible wavelengths and with thin shells (<15 nm) feature an unexpected color effect. Single particle and powder spectroscopy, as well as calculations based on Mie theory were used to investigate this phenomenon. The use of HSNPs increases the transport mean free path of light significantly, which reduces multiple scattering, and thus the Mie resonances become visible to the bare eye.

Measuring single small molecule binding via rupture forces of a split aptamer.

The rupture force of a split (bipartite) aptamer that forms binding pockets for adenosine monophosphate (AMP) was measured by atomic force spectroscopy. Changes in the rupture force were observed in the presence of AMP, while this effect was absent when mutant aptamers or inosine were used. Thus, changes in the rupture force were a direct consequence of specific binding of AMP to the split aptamer. The split aptamer concept allowed the detection of nonlabeled AMP and enabled us to determine the dissociation constant on a single-molecule level.

Confined Liquid: Simultaneous Observation of a Molecularly Layered Structure and Hydrodynamic Slip

The force profile between a glass microsphere and mica in 1-propanol has been measured with the colloidal probe technique. Oscillatory solvation forces indicate a layered structure of the confined propanol for at least three layers. In the same experiment, hydrodynamic forces were measured at high approaching velocity. Comparing measured force curves with calculations we found a significant effective slip, which could be described by a slip length of 10–14 nm.

Confined Diffusion in Periodic Porous Nanostructures

We performed fluorescence correlation spectroscopy measurements to assess the long-time self-diffusion of a variety of spherical tracer particles in periodic porous nanostructures. Inverse opal structures with variable cavity sizes and openings in the nanometer domain were employed as the model system. We obtained both the exponent of the scaling relation between mean-square displacement and time and the slow-down factors due to the periodic confinement for a number of particle sizes and confining characteristics. In addition, we carried out Brownian dynamics simulations to model the experimental conditions. Good agreement between experimental and simulation results has been obtained regarding the slow-down factor. Fickian diffusion is predicted and seen in almost all experimental systems, while apparent non-Fickian exponents that show up for two strongly confined systems are attributed to polydispersity of the cavity openings. The utility of confining periodic porous nanostructures holds promise toward understanding of constrained diffusion with a wide range of applications ranging from water purification and drug delivery to tissue engineering.

Transition in the Evaporation Kinetics of Water Microdrops on Hydrophilic Surfaces

We describe a technique that allows measurement of the mass and shape of sessile liquid microdrops during evaporation. Therefore, the microdrops are deposited by an inkjet onto a silicon microcantilever, and the bending and the shift in resonance frequency are monitored. From hydrophobized surfaces, microscopic water drops evaporate with the same kinetics as macroscopic drops; we verify the validity of known evaporation laws to drops with diameters from 100 microm to below 10 microm. From hydrophilic surfaces, the evaporation is slowed down during the last approximately 100 ms; we believe that this occurs due to flattening of the drops, which are then stabilized by interfacial forces and disjoining pressure.

Detachment Force of Particles from Air-Liquid Interfaces of Films and Bubbles

The detachment force required to pull a microparticle from an air-liquid interface is measured using atomic force microscopy (AFM) and the colloidal probe technique. Water, solutions of sodium dodecyl sulfate (SDS), and silicone oils are tested in order to study the effects of surface tension and viscosity. Two different liquid geometries are considered: the air-liquid interface of a bubble and a liquid film on a solid substrate. It was shown that detaching particles from liquid films is fundamentally different than from bubbles or drops due to the restricted flow of the liquid phase. Additional force is required to detach a particle from a film, and the maximum force during detachment is not necessarily at the position where the particle breaks away from the interface (as seen in bubble or drop systems). This is due to the dynamics of meniscus formation and viscous effects, which must be considered if the liquid is constrained in a film. The magnitude of these effects is related to the liquid viscosity, film thickness, and detachment speed.

Light Induced Charging of Polymer Functionalized Nanorods

ZnO nanorods were functionalized with new block copolymers containing a hole transporting moiety in one block and a dye and an anchor system in the second block. After functionalization, the ZnO nanorods are well dispersible in organic media and the fluorescence of the dye is quenched. Kelvin probe force microscopy was used to measure changes in electrical potential between the ZnO nanorod and the polymeric corona. Upon light irradiation, potential changes on the order of some tens of millivolts were observed on individual structures. This effect is attributed to light-induced charge separation between the ZnO nanorod and its hole transporting polymeric corona.

Interfacial forces between a silica particle and phosphatidylcholine monolayers at the air-water interface.

Interfacial forces between a silica or borosilicate particle in water and phospholipids at the air-water interface were studied using the Monolayer Particle Interaction Apparatus. This instrument allowed the forces to be measured as the colloidal probe approached the monolayer from the liquid phase. The proper working principle of this setup was demonstrated by measuring the forces between a particle and a mica plate in 0.1 mM NaCl. The effect of the alkyl chain length on the adhesion between the particle and the monolayer was investigated using four different 1,2-dialkyl-sn-glycero-3-phosphocholines (DMPC, DPPC, DSPC, and DBPC), which had 14, 16, 18, and 22 carbon atoms per alkyl chain, respectively. The adhesion force increased with the square of the particle radius. The lipids in the liquid-expanded (LE) phase showed an attraction to the particle, explained by an electrostatic attraction and/or the formation of a three-phase contact line that lead to a capillary force. All monolayers showed an adhesion in their retract force curve, which decreased with an increased chain length and surface pressure. Interfacial stiffness was generally seen to increase with the phospholipid chain length and to decrease with surface pressure, explained by an increase in the intermolecular van der Waals interaction and a decrease in the interfacial tension, respectively. The adhesion between the particle and monolayer was concluded to be controlled by the contact area between the particle and monolayer, and therefore the monolayer stiffness and the electrostatic interactions.