Low Liquid-Liquid IFT Measurements
Liquid-vapor interfacial tension (IFT) measurements are easily carried out with traditional pendant drops. These are a millimeter or two tall and have a similar millimeter or so diameter for liquid-vapor systems, most of which have IFT's in the 5 to 100 range.
Liquid-liquid measurements are difficult only in the sense that the size of the suitably-shaped pendant drop varies over a far wider range. The IFT between the two liquids can approach zero, as can the density difference. Both of these parameters are important in determining the stability and size of the resulting drop.
With a suitably equipped instrument, the limiting factor is the nature of the liquid-liquid system. The following is a list of factors to consider.
The miscibility of the liquids is the most important single issue. Do they mix? You can test your liquids easily by pouring a small amount of one into another. Do they stay mixed or quickly separate into two layers? The time they take to separate gives you an indication of the difficulty of forming a stable pendant drop with the two liquids. A few seconds is tolerable but anything longer than a minute should be viewed with suspicion. The ease with which they separate depends both on their chemistry (their IFT) and their density difference.
If you have a difficult system, you may want to mutually saturate them before trying the above test. This consists of pouring a little of each into the other and then letting them stand overnight. Then retry the separation test. By the way, the liquids should be mutually saturated before conducting real measurements also.
The liquids must have differing refractive indices in order that the pendant drop appear dark in the image. Most liquids have a sufficiently different index that this is not a problem.
Liquids with a low density difference will have a relatively tall drop, all other things being equal. If the drop is large, it will be particularly subject to vibration and an air table may be necessary.
The overall size of the pendant drop depends on both the density difference and the IFT between the liquids. This is discussed in an FTA application note. Since the final drop may be either large or small, zoom optics are a necessity so the operator can make the final drop fill the image.
The drop must be sufficiently tall that the shape is no longer round, but rather is pendant-like. The larger the IFT, the more round (un-pendant-like) the drop will be for any height. Thus
- small IFT's lead to small drops being OK
- small density differences lead to larger drops being required
Therefore, there is a contest when both the IFT and the density difference are small. A drop of the required shape might be quite small or quite large. The table in the above reference helps you estimate the final drop size.
For a constant size, the density difference and IFT are proportional. If the density difference is 1/5, say, the IFT will have to be 1/5 to maintain the same size. By selecting a proposed size, you can relate density difference and IFT. Try different drop sizes (up and down in the table), and scale by IFT and density difference, until you find a combination that is close to your system.
Viscosity is not normally a difficult issue. Higher viscosity slows down time to form a drop, but does not affect the final drop. If you can form a drop using the pipette protocol described below, viscosity is not an issue.
Please note that the IFT between two liquids is not the difference in their individual liquid-vapor IFT's. The liquid-liquid IFT must be measured.
As a general rule, liquids with an IFT of 0.01mN/m or less are very difficult to measure. Difficult does not mean impossible, but it does mean not easy. This is not an instrument issue. It is a question of the liquids. Can you form a stable pendant drop of one in another? You can answer this with a simple pipette and a stand to position it over a beaker. Fill the beaker with the lower density liquid and the pipette with the higher density. Form a pendant drop and slowly dip it into the liquid of the beaker. Does it remain intact, or does the pendant drop spread out and mix with the beaker's liquid? When the IFT is down in the 0.01 range, it says the liquids are well on the way to being miscible anyway.
There is a final backup when the liquids do not form a really stable drop, but do have some finite IFT. This backup is to use the Drop Weight method. The accuracy is low, of the order of 10%, but sometimes this is all that is possible. It attempts to measure the volume of liquid drops detaching from the dispense tip, as they will when a stable drop can not be formed.