A fully featured version (n = c) has been developed for the Apple Macintosh. This software is inexpensive, and available for purchase from the Department.
A free demonstration version (n = b), is available on request to the address below.
Software Sales enquiries to: Surface.Forces@rsphysse.anu.edu.au
"Non-LinearPB Apl" is written in complied FutureBASIC 3. The program has been tested on Mac II etc., Centris, Quadra, LC 475, PPC 7200 and iMac computers running Systems OS7 and OS8 and OS9; it will not run with OS6. If the program is being used illegitimately the user will be informed by an alert. The program is supplied in FAT version i.e. it will run optimally with 68*** and PPC processors.
The purchaser of this program undertakes that it will be used
only under the conditions of the license agreement on only one
computer at a time and to make copies of the program only for the
purpose of back-up. The program may be disposed of only if the person
or body receiving it agrees to abide by the same conditions.
Copyright of the program remains with the Australian National
University.
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Instruction Manual
1. Purpose of the
program
The "Non-LinearPB Apl" program calculates the normalised force F/R (where F is the force and R is the effective radius of the surfaces) as a function of nearest separation D between identical surfaces immersed in a symmetric n:n liquid electrolyte. The force is the sum of the attractive van der Waals force and the repulsive force caused by the electric double layer. The repulsive force is obtained from the non-linear Poisson-Boltzmann equation using the algorithm of Chan, Pashley and White [1]. The parameter R is the geometric mean of the principal curvatures between the surfaces [2]; e.g. for crossed cylinders of radii Rc then R = Rc, for two spheres of radii Rs then R = Rs/4, for one sphere of radius Rs and a plane R = Rs/2. Data may be displayed on the screen from files read into the program, and data and fits may be read out into an external file. "Non-LinearPB Apl" does not fit data automatically. Fits to data are done manually by changing fitting parameters and generating new curves to be displayed on the screen. A point to be noted is that, because of the nature of the algorithm [2], the force is the independent variable and the distance the dependent variable that is calculated. The program also gives as output the pressure in the electrolyte between parallel surfaces without any contribution from the van der Waals force. Papers in which the program has been used to fit data are [3,4,5].
2. To start the program
To launch the program double click its icon. An empty graphsheet and a menu bar will be presented from which desired actions may be selected.
3. Units
S.I. units are used. Normalised force is in milli-newtons per metre (mN/m), distances are in nanometres (nm). Concentration is in Moles/litre, Surface charge is in Coulombs per square metre (C/m^2), Area charge in square nanometres per electronic charge (nm^2/e). The Hamaker constant is in Joules, for example the Hamaker constant for mica-water-mica is - 2.2E-20 J; see [5] for further discussion of the van der Waals force. The surface potential (assumed to be the same for both surfaces) is in millivolts (mV) and must be positive.
4. File Formats
Input and output files have the first parameter DISTANCE (in nm), second parameter FORCE (in fact F/R in mN/m) and the third PRESSURE (in MegaPascals). One triplet of parameters is on each line of the output file. The file format is that of a text file with the parameters separated by a delimiter which may be chosen to be a comma or a Tab, the Tab being useful for spreadsheets. It is only necessary to specifically set the delimiter, which is chosen from the File-Data Delimiter menu, for output files; in input files either format will be read in automatically. The Save (output) file contains the fitting parameters as well. Two specimen files of input data "Trialdata-TAB" and "Trialdata-comma" are provided for demonstration purposes. The demonstration version of the program will open only the data file "Trialdata-TAB" whatever file is requested and is not able to save data to disk. This restriction does not apply to the fully featured version.
5. Menus
a) Apple Menu: Owner of program.
b) File Menu:
Open File for Save: This loads one file with up to 900 points which is drawn on the screen, saved and outputted together with the calculated data. The fit is carried out by adjusting the fit parameters to make the calculated force curve as close as possible to the data points.
Open FileS Not for Save: loads up to five files with up to 900 points which will be drawn on the screen but not be saved.
Data delimiter: choose either Tab (default) or comma as delimiter between data points in loaded and saved files.
Quit: quit program and return to desktop. The program may also be terminated by clicking the close box.
c) Calculation Menu:
Parameters: The parameters used for the calculation are set in this dialog box. When the program is first invoked default values are presented. The most important of the parameters are the (Surface) Potential and the Debye length. To calculate Debye length from concentration set the Debye Length to zero, otherwise the concentration is calculated from the Debye length (Concentration must be set to zero). Details of the other parameters are given at the end of this manual. A calculation at constant surface potential or constant surface charge may be chosen by clicking on the square boxes. If the numerical values of the parameters are not visible in the relevant box select the visible region and move the cursor with the arrow keys.
Calculate Now: This starts the calculation. A list of parameters used in the calculation is displayed on the screen and the calculation starts and is displayed step by step. The calculation may be aborted at any time by clicking the mouse button.
d) Plot Menu:
Format: This menu determines the scale of the plot displayed and whether the vertical scale is linear or logarithmic. The distance scale is always linear.
Default: This restores the default values of the plot scales.
Zoom: Create a zoom box by pressing the mouse button. The cursor will change from an arrow to a cross. Dragging the mouse cursor in the bottom right direction with the mouse button down will create a movable box. The box will disappear if the cursor is moved off the window to the top or left. Releasing the mouse button fixes the zoom box. Selecting the Format-Zoom menu scales the plot to the zoom box. More than one box may be created; it is the most recent that is zoomed.
e) Help: displays some helpful information. Dismiss this screen by clicking the mouse button.
To refresh the screen use the Plot-Refresh menu.
6. Details of Calculation Parameters
Potential (mV): The surface potential. It is the same for both surfaces (assumed symmetric) and must be positive. In the case of a constant surface charge calculation this is the potential when the surfaces are far apart.
Constant Charge:
Constant Potential: Click the box to select which condition is desired.
Debye Lngth (nm):
Concentration (moles per litre): If Concentration is set at zero, the Concentration is calculated from input value of Debye length. If the Debye length is set to zero, the Debye length is calculated from input value of Concentration.
Layer Thickness (nm): distance offset for electric force. Positive value gives distance of origin of van der Waals force behind electric surface charge.
Temperature (K) of electrolyte.
Static dielectric Constant of solvent (78.36 for water).
Electrolyte n:n. charge number of symmetrical electrolyte. Allowed values are integers n = 1 to 4.
F/R: = Hamaker Constant of van der Waals force, geometry coefficient, power law
Exponential force: coefficient, exponent of an exponential force that may be added to describe hydrophobic forces etc.
Step number: number of pressure steps in calculation. see [1]. Increase this to obtain more fit points at long distances; at very large numbers of steps it may be necessary to increase the number of Runge-Kutta steps too. The step number should be large enough so that the force at the first step is a small fraction of the forces to be fitted. At short distances the distance between points is determined by - Distance Increment.
Distance Increment: minimum distance between points at short distances.
Minimum distance (nm): The calculation starts at infinite separation and stops when this distance is reached.
Number of steps in Runge Kutta integration: Default is 25. Surprisingly accurate results may sometimes be obtained for fewer steps but in other cases a larger number of steps may be required. The appropriate criterion is that the number of Runge-Kutta steps should be large enough so that the output data do not depend significantly on the number of steps. Varying the number of steps has most effect at long distances.
If any of the calculation parameters are out of range the user will be informed by an alert and control will be returned to the appropriate parameters dialog box.
[2] A M Stewart. J. Coll. Inter. Sci. 170, 287 (1995).
[3] A M Stewart. Measurement Science and Technology 8, 114 (1995).
[4] A M Stewart. Measurement Science and Technology 11, 298 (2000).
[5] V V Yaminsky, B W Ninham and R M Pashley Langmuir 14 3223 (1998).
[6] V E Shubin and P Kekicheff. Journal of Colloid and Interface Science 155, 108 (1993).
Links: Visit our Surface Forces Apparatus web site.