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Surface and interface characterisation

Dynamic Contact Angle measurement

The contact angle is the angle at which a liquid/vapor interface meets the solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Using the DSA from Krüss GmbH we probe the surface using a variety of test liquids to obtain contact angles and subsequently the surface energy of the sample material. Hydrated (or wet) samples can also be characterised using the captive bubble techniqueAdvancing andreceding angles can be meaured by expanding and contractnig the test liquid (or air) on the substrate or by tilting the table, which also allows to determine sliding angles. Hot melts and their interaction with surfaces (such as powder beds) can also be studied up to tempeatures of 400°C.

Contact angle of polymer melts

We have accessories for the Krüss DSA 10 that allow polymers to be extruded/injected (Fig. 1) into an environmental, temperature controlled measurement chamber (Fig. 2). The chamber can also be used to assess contact angles between a polymer melt and a substrate (in an inert/chosen gas atmosphere). Imbibition measurements of polymer melts into powder compacts of the reinforcement phase can be made to allow assessment of the interface between the phases.

Fig 1. Extruder for Krüss DSA contact angle machine (left), and Fig 2. Environmental chamber for the contact angle machine (right)

High pressure and temperature contact angle rig

Surface and interfacial tensions (i.e. contact angles, captive bubbles/drops, interfacial tensions) at elevated tempartures and pressures are required in the development of materials or the understanding of phenomena relevant to the oil and gas industry.

The surface (pendant drop) or interfacial tension (pendant or standing drop) and wettability of a substrate in contact with one or two equilibrium liquid phases can be measured in a thermostatted high-pressure view cell in aggressive environments at pressures of up to 700 bar at temperatures up to 200°C. The pure liquid components can be injected into the previously evacuated cell in known amounts; the first using ordinary syringes and the third from a high-pressure syringe pump which will also serve to set the experimental pressure. The contents of the cell are brought into phase equilibrium at the set temperature with using mechanical agitation, and the phases will then be allowed to separate. A droplet of the lower (more-dense) phase can be injected into the top of the cell or the upper (less-dense) phase into the bottom of the cell. This permits a pendant of standing drop to be created which is imaged for the determination of the interfacial tension. Alternatively, the cell can be fitted with a substrate onto which one or other phase can be injected for the investigation of wettability.
Density measurements are made on gravimetrically-prepared mixtures under single-phase conditions by means of a commercial high-pressure vibrating U-tube densitometer (DMA 512P, Anton Paar).

Zeta Potential (streaming and electrophoresis) measurement

The zeta-potential of porous media, fibres, films, nanotubes and powders can be studied using various measuring cells by the streaming potential method (Fig 3). We have two EKA Elekrokinetic Analysers (Anton Paar, Austria) and perform measurements as a function of time, pH and type and concentration of the electrolyte to assess both the stability of the measurement and the acid-base surface characteristics. The plateau region and the isoelectric point (point of zero charge) can be determined by pH dependent zeta-potential measurement. We can perfrom in situ measurements of samples to assess surafce modification by (for example) plasma treatment or protein adsorption. The machine can also be used to determine the permeability of porous materials.

Fig 3. Streaming zeta potential instrument (left), and Fig 4. Surpass (right)

Tensiometry and single fibre wettability measurement

The dynamic fluid sorption (capillary rise) technique is used to determine the wettability of porous systems, fibres, powders and membranes using an automated tensiometer (Krüss K100) (shown inFig 5). Using the modified Washburn equation enables to determine the surface enegry of such solids. The amount of liquid retained after measurment is also determined. Furthermore this tensiometer allows for the determination of surface and interfacial tensions using the Ring- or Wilhelmy-plate methods. The density of liquids can be determined using Archimedes principle using this instrument, this machine also allows the measurement of sedimentation rates of suspensions. We modified an ultramicro balance (4504 MP8, Sartorius, Göttingen, Germany; accuracy = 0.1 μg) to enable us to measure contact angles on single (carbon) fibres with diameters as low as 5 μm using the modified Wilhelmy-technique (shown in Fig 6).

Fig 5. Tensiometry machine (left), and Fig 6. Single fibre wettability measurement (right)

Microscopic analysis

We have a wide range of optical microscopes, which are interfaced with digital cameras and image analysissoftware for measuring (e.g. pore sizes). We have Linkham hot plate and cold stage accessories for studying the development of crystallini ty of polymers over time, this is also used to identify the site of nucleation (i.e. from the surface of carbon fibres). We also have access toScanning Electron and Transmission Elctron Microscopy in the Department of Materials.

BET Surface Analysis

We co-own a BET surface area analyser which is situtated in the department's analytical services laboratory. The surface area can be attained by the BET isotherm method, whereby nitrogen is adsorbed and subsequently desorbed. The size of a nitrogen molecule is known, therefore by monitoring the mass change over time, the surface area of a known mass of sample can be attained.

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