DE20209563U1 - Apparatus used in electro-kinetic analysis e.g. in the manufacture of paper and fiber board, comprises oscillating liquid flow produced in such a way that small sample and liquid volumes can be processed - Google Patents

Abstract

An apparatus comprises an oscillating liquid flow produced in such a way that small sample and liquid volumes can be processed. The ratio of the liquid volume to the packing volume of the solid sample is not more than 10: 1 for fibrous, powdered or granulated sample material. The ratio of the liquid volume to the fixed surface is not more than 0.5 cm3>/cm2> for planar samples.

Description

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Prof. Dr. Robert Kohler

Stettiner Str. 21

72116 Moessingen

Phone: 07473/24535

e-mail: robert.kohler@fh-reutlingen.de

Title: Electrokinetic analysis with minimal liquid volume (MINELKA)

Description Summary:

The characterization of surfaces and interfaces and the interactions that take place there is a central topic in materials research and is important for a wide range of issues, e.g. in chemistry, biotechnology and medical technology. Electrical effects at solid-liquid phase boundaries, the electrical double layers and the associated zeta potential of the solid, are characteristic of the respective material and its current environment. The electrical potential of the solid surface influences the adsorption and adhesion of substances from the environment.

The zeta potential of a solid is usually determined using the streaming potential or the streaming current for a stationary solid and a moving liquid. The alternative measurement of the electroosmotic effect has not been used to date due to experimental difficulties. The known commercial devices have the disadvantage that they have to work with relatively large amounts of liquid (approx. 0.8 liters) and a very small solid/liquid ratio (FFV) of approx. 1:50 - 1:100. The task is therefore to develop a device that can be operated with very little liquid and an FFV of < 1:10. This then allows measurements to be made with real, limited (e.g. physiological) liquids and enables the investigation of dynamic processes such as adsorption/desorption.

The basis of the measuring device is a programmable linear motor that can move the liquid with almost any adjustable path-time oscillations. The pressure is measured directly in the drive piston. Measuring cells can be used to suit the respective task. In this specific case, one for dispersed solids such as powder or fibers, a second for flat samples such as foils or plates. In addition to the measuring channels for flow potential and flow current, specially developed electronics also have an input for the cell temperature and two channels for recording the internal and, if necessary, external pressure. Conductivity and pH are recorded using a separate measuring device that can also control a dosing unit.

&bull; The oscillation principle underlying the device enables measurements from a minimum usable liquid quantity of approx. 8 ml and an FFV of approx. 1:8. This makes it possible to track dynamic processes at the interface that were previously not detectable electrokinetically.

&bull; The device can perform both streaming potential and electroosmosis measurements on the same sample.

&bull; The preparation and measurement time for a single measurement is reduced from more than 50 minutes for a commercial device to less than 10 minutes.

Brief description of development and results

Examples of current problems are:

&bull; Characterization of fibers with regard to adhesion and reinforcing effect in composite materials, behavior in textile finishing processes, use in filter materials, for the production of paper and fiberboard, etc.

&bull; Investigation of polymers, as matrix material for composites, for adhesives or for specific applications,

&bull; Investigations on powders or granules, e.g. for the production of high-performance ceramics, sintered bodies, filters, etc.

&bull; Investigation and characterization of biomaterials with regard to biocompatibility, performance and stability.

&bull; Biomedical examination of organs and tissue, e.g. processes in the movement of body fluids, electroosmotic effects when loading bones, etc.

&bull; Movement of fluids in geological layers (sand, rock) with regard to groundwater issues or oil extraction.

&bull; Electroosmotic effects in seismic processes, e.g. in earthquake research.

Biomaterials and implants require comprehensive investigations of their surface properties. The task therefore is to create new analytical possibilities.

Especially for biomedical questions, the devices known to date or those available for sale are only of limited use, as large quantities of liquid are required for the measurements. This rules out the examination of very small samples in a real environment, and a suitable device for this is not yet available on the market.

The aim is therefore to develop an innovative measuring device for minimal sample and liquid volumes that can be applied to real systems and difficult media (organic, biological) and that provides additional information in parallel to the actual electrokinetic analysis.

A measuring device has been developed that achieves these goals and provides new, previously unavailable examination options.

Device description

The basis of the newly developed measuring device is the application of an oscillation principle.

An interesting indication of the timeliness of this project is that during its lifetime several new papers have appeared on the use of oscillating electrokinetic measurements in fields as diverse as oil exploration. However, none of the known publications addresses the issue of minimizing sample size.

In the measuring device implemented, a minimal amount of liquid is passed in an oscillating manner past the solid surface to be examined and the resulting flow potential or the corresponding flow current is measured together with the resulting differential pressure. At the same time, the electrical conductivity, pH value and temperature can be recorded in the liquid. See Figure 1.

An elegant solution for generating variable fluid flows was found by using a fully programmable linear motor (1) that moves a piston (3) guided in a graduated precision cylinder (2).

Different measuring cells (4) can be placed on or in the cylinder. In the simplest form, the arrangement is vertical, the measuring cells are open at the top and the liquid is moved against the external air pressure. This makes it very easy to insert measuring probes (6) and to add any amount to the measuring liquid.

It is crucial that the pressure sensor (7) is located directly on the front side of the moving piston, which ensures a close coupling of piston movement and pressure signal and avoids dead volume. The pressure sensor also contains a PTIOO temperature sensor.

Although a one-sided open measuring system is particularly simple, the possibilities are not limited. For denser samples that require higher pressure differences, two pistons are required, each containing a pressure sensor.

The electrokinetic signals are picked up with electrodes (8) (Ag/AgCI or Pt electrodes) and fed to a measuring electronics (9) with a sufficiently high input impedance. Special electronics were newly developed for this central task. The measured voltages are in the millivolt range, the currents in the range of micro- to nanoamperes.

Initially, two types of measuring cells were built. One for the investigation of fibers, see Figure 1, where the fiber sample (10) is placed between two sieve electrodes (8) and the electrolyte solution (5) is moved through the sample (10).

The second measuring cell is suitable for the flat samples of the implant materials (see Figure 2). Two sample plates (11) are attached parallel in the cell,

so that a narrow channel is open between them through which the electrolyte solution can flow during the cyclic movements of the plunger (3).

The device developed as part of the project enables measurements with liquid volumes of around 8 ml or more. This reduction in sample and liquid volume allows the investigation of substances that are only available in limited quantities. The minimum volume of the moving liquid column results from the geometry of the measuring cells and the requirement that the sample and the measuring sensors have full contact with the liquid at all times and that a sufficient flow rate is still achieved. In the selected design, the minimum diameter of the pump piston is limited by the pressure sensor.

A further reduction of the measurement volume is possible in principle, but would not bring any further advantages for the tasks at hand, but would only increase the difficulties. As the volume is reduced, the specific surface area of the cell increases in comparison to the sample material, resulting in an increasing overlap of information.

By reducing the measuring volume, the ratio of liquid volume to sample volume (or sample surface) could be reduced to such an extent that surface reactions such as adsorption, desorption, dissociation and dissolution can be recorded as detectable concentration changes in the liquid phase and can also be tracked kinetically. In the fiber cell, the solid/liquid ratio can be reduced to less than 8:1. In the plate measuring cell, the surface to volume ratio is crucial; it can be reduced to approx. 2 cm ² /cm ³ .

The use of a precision cylinder and the precisely controlled piston movement allow the sample geometry (cross section and length) to be set precisely, as well as the relationship between volume flow and pressure to be recorded precisely. In addition, with the selected arrangement, both the flow potential / flow current and the electroosmotic effect can be measured on one and the same sample by simply switching.

Any desired path-time profile can be realized with the programmable linear motor. An example of a linear motion profile and the resulting pressure curve are shown in Figure 4 a) and b).

The flow velocities or pressure differences that are decisive for the electrokinetic effects are obtained by differentiating the path-time function of the piston movement. For most tasks, a sinusoidal movement is appropriate, which thus causes a cosinusoidal pressure curve. However, the option of other movement profiles opens up additional, interesting investigation variants. An example of this is shown in Figure 5 a) and b).

A very compact combination electrode is used to measure the pH value and conductivity of the contact liquids, with which both variables are recorded simultaneously. This electrode also contains a temperature sensor required for temperature correction and also allows the control of an automatic titration device.

Temperature differences are measured very precisely by arranging temperature sensors on* both sides of the sample.

All functions are controlled and data is collected via a PC.

A multi-channel data acquisition system was used for data acquisition, which, due to its high acquisition rate, allows the individual measured values to be recorded practically simultaneously. The recorded data is transferred to a PC for further processing and evaluation. The Auto-Signal™ program, AISN Software Inc., is used for specific signal evaluation routines (e.g. smoothing, normalization and Fourier transformation).

Description of the measuring device

A schematic of the device is shown in Figure 3.

The system has a modular structure.

1. Measuring arrangement: The cell body (A), which is open at the top in the prototype, is pushed tightly onto a graduated precision glass cylinder. The cell body accommodates the actual measuring cells, two measuring electrodes (B) and the sample (P). The electrokinetic signals are transmitted from the electrodes to the "Minelka electronics" (C) and/or another measuring device with high input impedance.

The cell body consists of an acrylic glass block with a central, cylindrical channel and one or two side mounts for the electrodes. At the lower end, two O-rings seal the precision cylinder. Different measuring cells can be inserted into the central channel.

2. Pump and drive: A programmable LinMot® P linear motor (D) moves a Teflon piston in a defined manner, which runs in the glass cylinder and generates the selected fluid oscillation. A pressure and temperature sensor (E) is located flush with the front of the pump piston and transmits its signals to the Minelka electronics (C).

3. Minelka electronics (C): This is the central measuring module. Two switchable amplifier channels with high input impedance record and amplify the flow potential or the flow current. In addition, 2 pressure signals and a temperature signal can be recorded.

4. Data acquisition module (F): The amplified analog signals are digitized in the DAQ pad, a multi-channel, programmable analog-digital converter, and fed to the PC via a USB interface. The DAQ pad also has a trigger output and two programmable outputs (R) that provide up to 10 volts DC or ±10 volts AC with adjustable frequency (1 - 50 Hz). These outputs can be used for electroosmosis experiments (EO), for example.

5. Electrochemistry module (G): A combination electrode (H) for pH, conductivity and temperature of the measuring medium is immersed in the measuring cell (A) from above.

The associated measuring electronics (O) also controls a titrator (I), which enables precise additions to the open cell via a capillary. The conductivity channel is also used to directly measure the cell resistance.

6. VEO (K):_For electroosmosis experiments, an amplifier is used here if necessary, provided the signal supplied by the DAQ pad (F) is not sufficient, or any other voltage or current source.

7. Keithley (L): The Keithley 2700 scanning multimeter, with high sensitivity and input impedance, is used here for additional measurements if required. It can be used in certain cases instead of the Minelka electronics, but due to the limited scan rate only for low-frequency oscillations (< 1 Hz). For electroosmosis measurements, the Keithley records the pressure signal.

8. Central computer (M): A PC centrally handles data acquisition and control of all system modules and functions.

9. Pressure sensor (N): is not needed for the one-sided open cell, it is intended for a measuring cell closed on both sides.

Results

The tests and measurements carried out so far show that the zeta potential values measured with the new device agree with the quasi-stationary values of the commercial EKA device, within the accuracy usual for electrokinetic measurements.

The key advantage of the new device is the reduction in the required sample and liquid volume, and the associated increase in the solid/liquid ratio. This enables measurements to be carried out that were previously not possible. The advantages of the new measuring device are:

&bull; Time saving: A simple measurement (pH and ionic strength constant) takes a maximum of 10 minutes, including loading and subsequent cleaning of the measuring cell. A corresponding measurement on the existing commercial device, on the other hand, takes at least an hour. A complete series of tests, with variation of ionic strength and pH value, which requires at least two working days with the commercial device, can now be carried out in 3 - 4 hours.

&bull; Sensitivity and robustness: Due to the positioning of the pressure sensor in the piston, the measurement signal follows even the smallest pressure fluctuations. Since the electrokinetic effect is proportional to the signal/pressure quotient, the measurement result is hardly influenced even by distorted oscillation movements. An example of the correlation of pressure and voltage even with a distorted sine function is shown in Figure 6. In fact, additional information about the higher frequencies can be obtained from the distorted signals using Fourier transformation.

Metal surfaces: With the new Minelka device, the electrokinetic examination of metal surfaces, especially titanium sheet, is easily possible (which has not been possible with commercial devices so far).

Electroosmosis: By simply switching electrically, both the flow potential/flow current and electroosmosis can be measured alternately on one and the same sample. To do this, an alternating voltage is applied and the resulting pressure fluctuations are measured. An example of an electroosmotic pressure signal is shown in Figure 7.

Tracking reactions in the liquid and/or on the surface: The possibility of time-controlled additions and the reduced liquid/solid volume ratio make it possible to track reactions over time. An example of the adsorption of a cationic polymer onto a titanium surface recorded with the device is shown in Figure 8. In addition, any properties of the liquid medium can be tracked over time simultaneously with the electrokinetic effects and the associated differential pressure. These are usually conductivity, pH value and temperature, but microscopic, optical and spectroscopic measurements are also possible.

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List of reference symbols

Figure 1: Fiber measuring cell with probes

(1) Linear motor

(2) Precision cylinder

(3) Piston

(4) Measuring cell

(5) Electrolyte solution

(6) Measuring probes

(7) Pressure sensor

(8) Sieve electrodes

(9) Measuring electronics

(10) Fiber sample

Figure 2: Plate measuring cell

(11) Test plates (e.g. metal plates)

(2) Precision cylinder

(3) Piston

(7) Pressure sensor

Figure 3: MINELKA measurement setup

(A) Measuring cell

(B) Measuring electrodes

(C) Minelka - Electronics

&Igr;&iacgr; :: '

(D) Linear motor LinMot® P

(E) Temperature sensor P1/T1

(F) Data acquisition module

(G) Electrochemistry - Module

(H) Combination electrode for pH, conductivity, temperature

(I) Titrator

(K) Amplifier for electroosmosis

(L) Keithley - Multimeter

(M) Central computer

(N) Pressure sensor P2

(O) Measuring electronics of the electrochemistry module

(P) Sample

(R) 2 programmable outputs

&bull;&bgr; ·

&bull; t ··

&bull; f ·

&bull;t ·

Claims (14)

1. Device for electrokinetic analysis with minimal liquid volume , characterized in that an oscillating liquid flow is generated in such a way that small sample and liquid volumes can be used, whereby in the case of fibrous, powdery or granulated sample material the ratio of the liquid volume to the packing volume of the solid sample must not be greater than 10:1, and in the case of planar samples the ratio of the liquid volume to the solid surface must not be greater than 0.5 cm ³ / cm ² . 2. Device according to claim 1, characterized in that parallel to the measurement of the electrokinetic effects by means of sensors, the differential pressure occurring at the sample, as well as the conductivity, the pH value, the temperature and optionally other properties of the liquid medium, are recorded simultaneously and in real time. 3. Device according to claim 1, characterized in that additives can be added to the measuring medium in a time-controlled manner, whereby the change in the electrokinetic effects and the properties of the liquid medium can be monitored simultaneously and in real time. 4. Device according to claim 1, characterized in that the effects measured in real time are used to control the dosing. 5. Device according to claims 1 to 4, characterized in that the liquid oscillation is generated by a precision movement such that the liquid transport taking place at any time is precisely known. 6. Device according to claims 1 to 5, characterized in that the liquid oscillation is generated by means of a direct linear drive, avoiding mechanical conversion of rotational movement into feed movement. 7. Device according to claims 1 to 6, characterized in that the liquid oscillation is generated by means of an electromagnetic or piezoelectric linear drive. 8. Device according to claims 1 to 7, characterized in that the direct linear drive enables any form of movement (distance-time functions), whereby positioning and speed are precisely determined at any time. 9. Device according to claims 1 to 8, characterized in that the direct linear drive generates a fluid oscillation in the frequency range 0.01 to 25 Hz. 10. Device according to claims 1 to 9, characterized in that the direct linear drive preferably generates a sinusoidal, occasionally a rectangular or a linearly progressive fluid flow. 11. Device according to claims 1 to 10, characterized in that the sensors required for pressure measurement are each positioned directly in the pressure-generating element. 12. Device according to claims 1 to 11, characterized in that the electronics for measuring the flow potential or the flow current have an active shield that dynamically follows the measuring signal to ensure the necessary high sensitivity and accuracy. 13. Device according to claims 1 to 12, characterized in that on one and the same sample, optionally either a mechanical liquid oscillation can be generated and the resulting flow potential (or the flow current) can be measured, or on the other hand, by applying an electrical alternating voltage, an electroosmotic liquid oscillation can be generated and the resulting electroosmotic effect can be measured. 14. Device according to claims 1 to 13, characterized in that the electroosmotic liquid oscillation is detected by means of the pressure sensors.