A team from the Physics Institute of the University of São Paulo (USP) has taken advantage of the properties of liquid crystals – substances that are half way between crystalline solids (crystals) and isotropic liquids (such as water) – to produce a piece of equipment that accurately calibrates hemodialysis machines, in order to make the filtering they carry out for people with defective kidneys more efficient. The group, coordinated by professor Antônio Martins Figueiredo Neto, has already filed a patent request with the National Industrial Property Institute (INPI in the Portuguese acronym) and it expects to find some manufacturers interested in producing it on a commercial scale.
The system consists of injecting a liquid crystal into the hose through which the blood flows and, through an optical cell, measuring changes in the speed of the flow. Once the speed is known, it can be established whether the machine is properly calibrated or not. Nowadays, hemodialysis machines are manually and inaccurately controlled, which may leave the flow of blood very fast, or very slow or with sharp changes in speed. “The device”, says Figueiredo, “can check the quality of the flow; which means whether the blood circulator to be tested causes sharp changes in the flow – which could lead to undesirable changes in blood pressure -, and the device can detect the problem and suggest changes to the moveable parts of the circulator to avoid this”.
The liquid crystals – a type of complex fluid – were discovered in 1888, when the Austrian botanist Friedrich Reinitzer was studying the properties of the cholesterol derivatives – acetate and Cholesteryl benzoate. He discovered that these substances, which at ambient temperature have a solid crystalline appearance, when heated, turn into a milky fluid. If they were heated further they changed again, this time into a transparent liquid. In the case of cholesterol benzoate, the two points of change occurred at 145.5°C and 178.5°C (degrees Centigrade). Since the phenomenon could arise because of possible impurities in the material, Reinitzer asked the German chemist Otto Lehmann to submit them to a chemical analysis to discover any possible contaminators “, says Figueiredo. “After careful study, Lehmann concluded that there were no contaminators: the cholesterol derivates were pure substances”.
How do we explain their behavior, therefore? The answer came in 1922 from the Frenchman George Friedel: the milky stage, which he called mesomorphic, corresponded to the then unknown liquid crystal, an intermediate state between the solid crystal and the isotropic liquid. “Another notable feature of these materials”, says Figueiredo, “is that they reflect white light selectively. Part of the radiation is absorbed and part is reflected, giving rise to colored light that varies with the temperature”. It was this property, analogous to that of crystalline solids, that led Lehmann to give the milky fluids the name of liquid.
And it was also what led the USP researchers to develop the equipment.”The liquid crystals”, says Figueiredo, “are grouped into two large families: the thermotropic, which tend to move toward or away from heat sources, and the lyotropic where the phase transitions are affected by changes in concentration and by changes in temperature”. The thermotropics account for 99% of applications – for example, digital watch displays, TV screens, and computer screens. But, as they are less studied and because of their specific properties, the lyotropics are more interesting: 60% of the work of the team making up Ifusp’s Complex Fluids Group is devoted to them.
The team’s project consisted of using linear and non-linear techniques and radiocrystallography to study the structure and properties of liquid crystals and ferrofluidic substances – another type of complex fluid. “We discovered, for example, that some lyotropics are optically isotropic – they have the same physical properties in all directions – when in a state of rest, but they become anisotropic when in movement”. Thus, in movement, their index of refraction varies, in order to let through more or less light, according to the speed. “It was based on this property that we invented the device enabling the working of hemodialysis equipment to be checked”.
The peculiarities of liquid crystals derive from the asymmetry in their molecules. Under certain conditions of concentration, pressure and temperature, the molecular axes can be steered in parallel to one another. It is this ordering that makes the liquid crystals behave a little like liquids, a little like crystalline solids and still be neither one thing nor the other. This is so as the molecules of a liquid are not arranged in any particular position or order, while the set of molecules of a solid crystal have a certain positional order. Liquid crystals have the fluidity of common liquids and, at the same time, the typical optical properties of solid crystals.
The molecules of thermotropics, which change phase according to temperature, are in the shape of a baton, a disk or a banana, all largely asymmetrical, or more precisely, anisotropic in shape. It is this anisotropy – the fact of the molecules not being arranged equally in all directions, like in a common liquid – that enables them to be ordered in respect of orientation. The basic components of lyotropics, on the other hand, which also change phase according to certain variations in concentration and temperature, are not made up of isolated molecules, but of molecular aggregations called micelles. The aggregations happen because their molecules are antagonistic – one part is polar and the other is apolar.
In contact with a polar solvent such as water – the molecules of which have an electrical dipole , in other words, positive and negative charges separated from one another, the micelles tend to take on a certain orientation; through electrical affinity, the polar region approaches the neighboring water molecule while the apolar region remains separated. From a critical concentration, the polar parts join themselves, like in a cocoon, within which the apolar parts remain separated from the aqueous environment. Each cocoon is a micelle and works as sealing between the apolar regions and the water. “This is what happens when we wash our hands to get rid of grease”.
When we have grease, an apolar material, on our hand and we try to wash with water alone, we cannot remove it. So we mix it with a detergent (whose molecules are opposites, with polar and apolar regions and we rub our hands, mixing the water, detergent and the grease. Now the detergent molecules make micellar type superstructures (cocoons) with the grease inside and they are washed away by the flow of the water”. Incidentally, the description of liquid crystals as pure substances applies to thermotropics, but not to lyotropics – which are, in fact, mixtures of at least two substances, that which forms the micelles and the solvent.
Another class of complex fluid the group studied is that of the ferrofluidic substances, with very interesting magneto-optical properties. They were invented in the 60’s by Nasa, to carry fuel from tanks to the engines of the space satellites. “The US space agency’s technicians produced a magnetic ingredient that could be dissolved in the fuel. They just had to apply low-intensity magnetic fields to conduct the material from one compartment to another, taking the fuel with it”, explains Figueiredo. To do this, they obtained a colloidal suspension with magnetite granules measuring about 10 nanometers , that could be dissolved in the fuel without the magnetic material being deposited on the bottom of the tank.
“The technological uses of the product later diversified. Nowadays, for example, it is used in the manufacture of magnetic paint to make aircraft invisible to radar, rotating seals that protect computer hard disks and in devices that detect the tilt of aircraft”.
Investigating the Optical and Magnetic Properties of Liquid Crystals (nº 96/09151-4); Modality Thematic project; Coordinator
Antônio Martins Figueiredo Neto – USP’s Physics Institute; Investment
R$ 31,000 and US$ 209,400