Elmar Bonaccurso

Boundary slip in Newtonian liquids: a review of experimental studies

For several centuries fluid dynamics studies have relied upon the assumption that when a liquid flows over a solid surface, the liquid molecules adjacent to the solid are stationary relative to the solid. This no-slip boundary condition (BC) has been applied successfully to model many macroscopic experiments, but has no microscopic justification. In recent years there has been an increased interest in determining the appropriate BCs for the flow of Newtonian liquids in confined geometries, partly due to exciting developments in the fields of microfluidic and microelectromechanical devices and partly because new and more sophisticated measurement techniques are now available. An increasing number of research groups now dedicate great attention to the study of the flow of liquids at solid interfaces, and as a result a large number of experimental, computational and theoretical studies have appeared in the literature. We provide here a review of experimental studies regarding the phenomenon of slip of Newtonian liquids at solid interfaces. We dedicate particular attention to the effects that factors such as surface roughness, wettability and the presence of gaseous layers might have on the measured interfacial slip. We also discuss how future studies might improve our understanding of hydrodynamic BCs and enable us to actively control liquid slip.

Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: An experimental investigation

Sessile liquid drops are predicted to deform an elastic surface onto which they are placed because of the combined action of the liquid surface tension at the periphery of the drop and the capillary pressure inside the drop. Here, we show for the first time the in situ experimental confirmation of the effect of capillary pressure on this deformation. We demonstrate micrometer-scale deformations made possible by using a low Youngs modulus material as an elastic surface. The experimental profiles of the deformed surfaces fit well the theoretical predictions for surfaces with a Youngs modulus between 25 and 340 kPa.

Superhydrophilic and superhydrophobic nanostructured surfaces via plasma treatment

Polyethylene terephthalate (PET) films have been structured with isolated nanofibrils and fibril bundles using oxidative plasma treatments with increasing etching ratios. The transition from fibrils to bundles was smooth and it was associated with a significant reduction in the overall top area fraction and with the development of a second organisation level at a larger length scale. This increased complexity was reflected in the surface properties. The surfaces with two-level substructures showed superhydrophilic and superhydrophobic properties depending on the surface chemistry. These properties were preserved during prolonged storage and resisted moderate mechanical stress. By combining different contact angle and drop impact measurements, the optimum surface design and plasma processing parameters for maximizing stability of the superhydrophobic or superhydrophilic properties of the PET films were identified.

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.

The Softer the Better: Fast Condensation on Soft Surfaces

Condensation on soft elastic surfaces differs significantly from condensation on hard surfaces. On polymeric substrates with varying cross-linking density, we investigate the nucleation and the growth of condensing water drops. With increasing softness of the substrates, we find (1) increasing nucleation density, (2) longer relaxation times for drop shape equilibration after merging of two drops, and (3) prevention of merging on very soft surfaces. These effects lead to higher surface coverage and overall condensed volume on soft surfaces.

Electrowetting -- from statics to dynamics.

More than one century ago, Lippmann found that capillary forces can be effectively controlled by external electrostatic forces. As a simple example, by applying a voltage between a conducting liquid droplet and the surface it is sitting on we are able to adjust the wetting angle of the drop. Since Lippmanns findings, electrocapillary phenomena - or electrowetting - have developed into a series of tools for manipulating microdroplets on solid surfaces, or small amounts of liquids in capillaries for microfluidic applications. In this article, we briefly review some recent progress of fundamental understanding of electrowetting and address some still unsolved issues. Specifically, we focus on static and dynamic electrowetting. In static electrowetting, we discuss some basic phenomena found in DC and AC electrowetting, and some theories about the origin of contact angle saturation. In dynamic electrowetting, we introduce some studies about this rather recent area. At last, we address some other capillary phenomena governed by electrostatics and we give an outlook that might stimulate further investigations on electrowetting.

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.

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.

Evaporation of sessile water/ethanol drops in a controlled environment

The evaporation of water/ethanol drops with different mixing ratios was investigated at controlled vapor pressure of water (relative humidity) and ethanol in the background gas. Therefore, a drop of about 1 microL was deposited on a hydrophobized silicon substrate at room temperature in a closed cell. With a microscope camera we monitored the contact angle, the volume and the contact radius of the drops as function of time. Pure water drops evaporated in constant contact angle mode. The evaporation rate of water decreased with increasing humidity. In mixed drops ethanol did not evaporate completely at first, but a fraction still remained in the drop until the end of evaporation. Depending on ethanol concentration in the drop and on relative humidity in the background gas, water vapor condensed at the beginning of the evaporation of mixed drops. Also, at a high vapor pressure of ethanol, ethanol condensed at the beginning of the evaporation. The presence of ethanol vapor accelerated the total evaporation time of water drops.

Measuring normal and friction forces acting on individual fine particles

Interparticle and surface forces are of great importance in many fields of pure and applied science. We present an apparatus to measure the normal and friction forces acting between a particle (radius of 0.5–20 μm) and another solid surface. The apparatus is based on the principle of an atomic force microscope. For quantitative friction measurements we propose a method to determine the lateral spring constants of atomic force microscope cantilevers with attached spherical particles.