Issue-27 (04/2012)

Foam under current – measuring the liquid content of foam

The measurement of foam stability by detecting the foam height is a well-proven method for foams with short-term stability. However, metastable foams require the investment of large amounts of time – differences between foam samples often only become obvious after several hours. In contrast, an indirect examination of the stability using the conductivity module PA4550 to measure the alteration in the liquid content is clearly more time-efficient. And it allows a correlation with other foam properties.

: Free choice: liquid content or foamability and foam decay – or all at once

Rule of thumb for foam: quick drying – quick decay

The decay of a foam is preceded by liquid drainage from the foam lamellas. The speed and extent of the drainage normally allows conclusions to be drawn about the stability of the foam – not as absolute values, but in relation to other samples. If the stability is derived indirectly from the drainage it is no longer necessary to wait for the actual and frequently very slow decay.

For numerous foam applications the liquid content and drainage not only provide an approach to the stability, but are also directly relevant. For example, the haptics of cosmetic foams or the mouthfeel of food foams depend substantially on their moisture content.

: Foam for body ...

: ... and soul

Fire extinguishers foams are even classified according to their moisture content, as this influences their fire-fighting effectiveness and discharge range. As a desirable side-effect, the drainage of the extinguisher foam also cools the material to be extinguished.

Information times seven

The Module PA4550 for the Dynamic Foam Analyzer DFA100 from KRÜSS measures the liquid content of the foam via its conductivity. If the conductivity of the foam is correlated with its initial mixture, the percentage fraction of liquid in the foam can be calculated. The measurement is made using electrodes arranged in parallel at seven different heights of the foam column, so that drainage is followed as a function of height. Foam generation is reproducible by means of a controlled flow of gas through a frit – the same foam generation method that the instrument otherwise uses for foam decay measurements via height detection. The software records independent measurement curves for all electrodes and gives the maximum moisture content as a percentage together with the half-value time for each measuring height; i.e. the time after which the initial moisture content has been reduced by half.

: Determining the liquid content at up to seven different heights

Simplicity itself: using the electrodes

The very comfortable handling of the electrodes is also new. They are arranged on two strips that are inserted in the standard measuring column of the DFA100 and connected up in only a few seconds. In the moisture determination measuring mode the standard foam height measurement is also carried out simultaneously, so that it is possible to change methods in no time at all, although in many cases no change is necessary.

The DFA100 and the Module PA4550 for determining the liquid content will be presented by KRÜSS this spring at the Analytica and at ACHEMA.

Mobile surface tension measurement:
the new Bubble Pressure Tensiometer BP50

Checking the concentration of cleaning agents on site or determining the right time for additional dosage of surfactants in the electroplating bath: this is the domain of the BP50. Mobile and independent of the mains, easy to use, and still a powerful measuring instrument for dynamic surface tension.

: Bubble Pressure Tensiometer BP50

Checking the surfactant content on site

Surfactants as wetting or cleaning agents change the surface tension. Measurement of the dynamic surface tension with the hand-held device BP50 reveals directly on site whether sufficient surfactant is dissolved. Concentrations above the critical micelle concentration (CMC), as is characteristic for cleaning baths, cannot be distinguished with static measurements of the surface tension. The dynamic bubble pressure method of the BP50 however reacts sensitively to changes in concentration even above the CMC.

Expert for high-speed

For high-speed processes such as printing, spraying or coating, it is the dynamic surface tension in the short-term that is decisive. The dynamic bubble pressure technique of the instrument detects the surface tension which occurs just after surface formation at surface ages between 15 and 16.000 ms with a resolution of 1 ms.

: A case for bubble pressure measurement: rapid surface formation with printing ...

: ... or spraying

Quality in hand

The measuring system, consisting of capillary, air supply, pressure sensor and a temperature sensor, is contained in a mobile hand-held instrument – for on-site measurements. Disposable plastic capillaries do away with the hydrophobizing process necessary for glass capillaries. Measuring curves from different capillaries have a very good reproducibility.

: LabDesk plot of surface tension as a function of surface age for a surfactant solution, measured with five different capillaries

The instrument works almost independently of the capillary immersion depth – a clear advantage for rapid tests. Water or other solvents can be used for calibration at different temperatures.

Proven LabDesk software with a new add-in

The measurement is defined and the instrument controlled by the BP50 add-in for LabDesk, the well-proven universal platform for KRÜSS tensiometers. Various measuring programmes record the time-dependent surface tension or the measured value curve for a constant surface age – or they evaluate a measured value as being valid or invalid in the rapid quality test. The database is comfortable and flexible; well-laid-out reports are quickly produced.

Overview of the BP50

Surface tension as a function of the surface age between 15 ms and 16 s
Built-in temperature sensor
Non-sensitive disposable plastic capillaries
Largely independent of immersion depth
Measuring programmes for surface age range, measuring series for constant surface age or individual test with quality assessment
Software for measurement definition and control with comfortable database together with plot and report functions

KRÜSS will present the BP50 for mobile measurement of the dynamic surface tension this spring at the Analytica and ACHEMA.

Like a rolling drop – internal tilting table for contact angle measuring instruments

Processes that we come across every day: rainwater pearling off from the windscreen or nano-impregnated umbrella, or the self-cleaning effect of hydrophobic outer walls or floor coverings. The fact that a drop adheres or rolls off tells us a lot about the properties of a surface.

Impregnation and self-cleaning place high demands on the coating
In order to study the roll-off behaviour, KRÜSS has developed the tilting table PA4240 / PA3240 for the contact angle measuring systems DSA30 and DSA100. It is possible to measure both the roll-off angle for a drop on a surface and the dynamic contact angle (advancing and receding angle) of a sliding drop. The latter provide information about the homogeneity and roughness of the solid surface.

: High demands on the coating:
Impregnation ...

: ... and self-cleaning (Picture: ITV Denkendorf)

Precision from horizontal to vertical

With the precise angle control to 0.1° for a large dynamic table movement range the roll-off angle and dynamic contact angle are determined reproducibly. The starting and finishing of the tilting movement are defined in the software and coordinated with the measurement. Samples up to 75 × 100 × 45 mm (L×W×H) can be tilted up to a vertical position. The drops can be deposited on a level or tilted sample.

Only a few actions are required to change from tilted measurements to the standard arrangement. For larger samples, or for the combination with horizontal axes, wafer tables and further accessories, tilting frame PA3220 is used together with the measuring system DSA100.

: Computer-controlled tilting and drop deposition

Wetting measurements for quality control: contact angles with DSA25

Plastics as ever-present materials and increasing trend to water-based bonding, coating and printing have made solving wetting problems an important challenge. Some time ago KRÜSS reacted to the increasing demand for simple wetting tests with the successful contact angle instrument EasyDrop. DSA25, its successor, offers enhanced possibilities.

: Drop Shape Analysis System DSA25

The DSA25 is a manual or semi-automatic first-user instrument for contact angle measuring techniques. As basic model, standard instrument or as expert version it provides options for differing demands.

DSA25B: Basic instrument with manual standard illumination, optics with fixed focal length, manual single dosing and contact angle module for drop shape analysis software DSA4. Simple extension to manual 2-fold dosing.
DSA25S: Standard instrument with software-controlled, high-performance illumination; 6.4× zoom optics with extended field of view and software-controlled single dosing, internal temperature measurement as well as software modules for contact angles and surface free energy. Simple extension to software-controlled 2-fold dosing.
DSA25E: Expert version; illumination and optics as for DSA25S, software-controlled 2-fold dosing; internal temperature measurement as well as software modules for contact angles, surface free energy and surface tension of liquids.

All versions are equipped with a high-resolution IEEE-1394 camera with 79 fps at full resolution. The camera viewing angle is set selectively using a scale. Extensive accessories such as thermostatted or humidity chambers or thermostatted dosing units can equip the instrument for solving particular problems.

The software DSA4 offers virtually everything for both newcomers and experts – from rapid single measurements up to programmed measuring sequences with almost unlimited numbers of drops.

Interfacial specialist for emulsions: the new Spinning Drop Tensiometer SITE100

Microemulsions are gaining ground wherever organic phases are to be mobilized by aqueous ones: for the bioavailability of drugs, tertiary crude oil exploitation, restoration of water, solvent-free degreasing or also for emulsion polymerization. The Tensiometer SITE100 from KRÜSS helps in the development of mini-emulsions and microemulsions as a specialist for extremely low interfacial tensions.

: SITE100 for very low interfacial tensions

Thanks to 25 year’s experience with spinning drop tensiometers, KRÜSS has been able to improve operating comfort even further: illumination and speed can be set by manual controllers or also via the software.

The instrument is equipped with a high-resolution USB camera and interchangeable lenses for various measuring ranges. Speeds of up to 20,000 rpm can be preset with great accuracy. Interfacial tensions down to 10^-6 mN/m can be reliably measured. The control and measurement of the temperature, which is important for emulsion properties, is just as accurate.

In addition to single measurements by mouse-click, the software also allows time-controlled series measurements. It saves the results in a comfortable database, together with their measuring parameters and drop images as well as the calibration data for the image scale. The plotting function plots the interfacial tension against time or the internally measured temperature. A final mouse-click, and the results are presented in a well-laid-out report.