What is the difference between a ferrofluid and a suspension

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If you are using our Services via a browser you can restrict, block or remove cookies through your web browser settings. We also use content and scripts from third parties that may use tracking technologies. You can selectively provide your consent below to allow such third party embeds. A magnetic colloid, also known as a ferrofluid FF , is a colloidal suspension of single-domain magnetic particles, with typical dimensions of about 10 nm , dispersed in a liquid carrier [].

The liquid carrier can be polar or nonpolar. Since the nineteen sixties, when these materials were initially synthesized, their technological applications did not stop to increase. Ferrofluids are different from the usual magnetorheological fluids MRF used for dampers, brakes and clutches, formed by micron sized particles dispersed in oil. In MRF the application of a magnetic field causes an enormous increase of the viscosity, so that, for strong enough fields, they may behave like a solid.

Ferrofluids are optically isotropic but, in the presence of an external magnetic field, exhibit induced birefringence [4]. Wetting of particular substrates can also induce birefringence in thin FF layers [5]. In order to avoid agglomeration, the magnetic particles have to be coated with a shell of an appropriate material. According to the coating, the FF's are classified into two main groups: surfacted SFF , if the coating is a surfactant molecule, and ionic IFF , if it is an electric shell.

There are essentially two methods to prepare these nanoparticles, by size reduction [1] and chemical precipitation [6]. In size reduction, magnetic powder of micron size is mixed with a solvent and a dispersant in a ball mill in order to grind for a period of several weeks.

Chemical precipitation is probably the most used method to prepare magnetic nanoparticles nowadays. Different procedures have been developed to achieve this goal. In general, these procedures start with a mixture of FeCl 2 and FeCl 3 and water. Co-precipitation occurs with the addition of ammonium hydroxide, and then the system is subjected to different procedures to peptization, magnetic separation, filtration and finally dilution.

Surfacted ferrofluids are formed by magnetic particles usually magnetite, Fe 3 O 4 coated with surfactant agents amphiphilic molecules, as oleic acid and aerosol sodium di-2 ethylhexyl-sulfosuccinate in order to prevent their aggregation.

Steric repulsion between particles acts as a physical barrier [7] that keeps grains in the solution and stabilizes the colloid. If the particles are dispersed in a nonpolar medium, as oil, one layer of surfactant is needed to form an external hydrophobic layer.

The polar head of the surfactant is attached to the surface of the particles and the carbonic chain is in contact with the fluid carrier. On the other hand, if the particles are dispersed in a polar medium, as water, a double surfactation of the particles is needed to form a hydrophilic layer around them.

The polar heads of surfactant molecules can be cationic, anionic or nonionic. In Fig. Surfacted ferrofluids are widely used in technological devices, being commercially available [8][9]. In ionic ferrofluids [10, 11], nanoparticles are electrically charged to keep the colloidal system stable. Usually, the liquid carrier is water, and the pH of the solution can vary from about 2 to 12, depending on the sign of the surface charge of the particles.

Ionic citrated or tartrated maghemite ferrofluids have both characteristics steric and electrostatic repulsion to prevent aggregation of the particles. In the presence of water, these attached molecules ionize. Besides the steric repulsion, there is also an electrostatic interaction. Magnetic particles display an almost spherical geometry, with a non-uniform shape distribution [15]. The distribution P D of diameters D , is usually given by a log-normal function [16, 17],.

The crystalline structure of the particles corresponds to the mineral spinel , MgAl 2 O 4 , in which divalent ions i. In this structure, the bcc primitive unit cell has 32 oxygen atoms, with 64 and 32 interstices of tetrahedral and octahedral symmetries, respectively. The stability of the magnetic colloid depends on the thermal contribution and on the balance between attractive van der Waals and dipole-dipole and repulsive steric and electrostatic interactions.

To evaluate the typical particle diameter D to avoid magnetic agglomeration we compare the thermal energy with the dipole-dipole pair energy [3] and get:. Plugging typical values in Eq. There are basically two main attractive interactions between magnetic particles in a ferrofluid, the van der Waals-London and the dipole-dipole interactions. The van der Waals-London interaction, U Aw , between two spherical particles of diameter D , separated by a distance r , is written as [20]:.

This is a short-range interaction and the attractive force increases with the particles size. In ionic ferrofluids, long-range electrostatic interactions between charged particles give rise to repulsive interactions, which guarantee colloidal stability.

On the other hand, in surfacted ferrofluids, there are steric repulsion forces, of short-range nature. Let us discuss in more details the case of IFF. As the bulk of this material is electrically neutral, there are counter-ions in the bulk of the suspension in order to compensate the surface charged particles. These counter-ions are driven to the surface of the particles, but are also subjected to electrostatic repulsion.

The calculation of the electrostatic repulsion between particles has to take into account this complex distribution of counter-ions. In the double-layer model [21], there is a first layer of counter-ions, called Stern layer, some angstroms thick, which involves the particle, and a second diffuse layer. These layers are separated by a Helmholtz plane. The interaction between two electrically charged spherical particles of diameter D , separated by a distance r , is written as [21]:. The case of steric repulsion is treated in some detail in Ref.

It turns out that this repulsion energy is linearly dependent on temperature. For spherical particles of diameter D , with a surfactant shell of thickness d and density x molecules per nm 2 , at temperature T one gets:. The behavior of the total interaction potential U T between particles, as a function of the interparticle distance r , is sketched in Fig.

Increasing further, it becomes dominated by the shielded Coulomb repulsion, in the case of IFF, or by the steric repulsion, in the case of SFF. For larger values of r see Fig. The most stable ferrofluids are designed so that the interparticle average distance to the nearest neighbors is approximately equal to that corresponding to the secondary minimum. Larger values of y o lower pH lead to higher potential barriers and smaller values of the depth of the secondary minimum.

The Figs. The typical size of the magnetic particles in a ferrofluid is on the order of 10 nm , sufficiently small for them to be magnetic monodomains. This is an important characteristic, because the particles have to have non-zero magnetic moments for the ferrofluid to show its magnetic properties. A fundamental property of the magnetic fluids is that, in presence of a non-homogeneous magnetic field, B r , they are attracted to the region where the field intensity is maximum.

B , which is parallel to the field. Two distinct mechanisms exist for the rotation of the magnetic moments in magnetic fluids. By lowering the temperature one comes to a temperature, T B , known as blocking temperature , bellow which t N is larger than the typical observation times. Bellow T B the particle is not anymore superparamagnetic, but the magnetic fluid is still superparamagnetic because the particle, and so also m , continues to be quasi-free to rotate.

Equations of motion for m , sufficiently general to be applicable for the cases of superparamagnetic and non-superparamagnetic particles, as well as mixed situations, where both mechanisms are important, can be found in the literature [22, 23]. Some hypothesis which are usually made in theoretical proposals for the rotational dynamics of the superparamagnetic particles and their magnetic moments in ferrofluids are:.

An example of computer simulation of the rotational dynamics of the particles in ferrofluids may be found in Ref. The thermodiffusion phenomena [] is particularly interesting in magnetic colloids. When this originally homogeneous material is subjected to a thermal gradient, there is a concentration current of magnetic particles parallel to the direction of the thermal gradient.

Thermodiffusion, also called Soret effect , is characterized by the Soret coefficient S T [27, ], which represents the coupling between current of mass and temperature gradient. A great effort was done by several groups around the world to improve the understanding of the thermomagnetophoretic mobility in ferrocolloids.

The Latvia group [] used the vertical column method to calculate the thermal diffusion coefficient from the measurement of the grains separation in the column. The experimental setup of the thermodiffusion column Fig. The temperature difference imposes convective flows at the walls in a way that an ascending stream is present near the warmer wall and a descending stream near the colder wall. Two separation chambers are placed at the ends of the column upper and lower parts where the grain's concentration are measured with the help of a LC oscillator.

Depending on the sign of the flux of matter along the gap the horizontal direction , ferrofluid grains will accumulate in the upper or lower chamber of the column. The french group used a different experimental technique to investigate thermodiffusion in ferrofluids, the Forced Rayleigh Scattering FRS. In experiments performed with the FRS technique, a thin colloid sample is placed in the interference pattern of two coherent intersecting pulsed pump laser beams Fig.

The space modulation of the light intensity generates by absorption processes, a modulation of temperature T which, in turn, generates a modulation of the nanograin volume-fraction f through the Soret effect.

T and f profiles are analyzed by diffracting a cw probe laser beam on this double-origin index grating. Lenglet and co-workers [37, 38] measured S T in different ferrofluids, obtaining values ranging from 10 -3 K -1 up to 10 -1 K Interestingly, the same temperature gradient gives positive or negative concentration gradients, depending on the particular ferrofluid under study.

In the ZS technique [40] a polarized chopped Gaussian laser beam, propagating in the z-direction, is focused to a narrow waist by using lens. The sample is moved along the z-direction through the focal point and the transmitted intensity is measured in the far field using a photodiode behind a small calibrated pinhole, as a function of the z-position. A chopper provides a square-wave light profile with a periodic succession of ON and OFF states of equal duration.

As the sample moves along the beam focus, self-focusing and defocusing modify the wave front phase, thereby modifying the detected beam intensity the setup is sketched in Fig. The variation of the index of refraction D n r , t , where r is the radial distance from the beam axis, can be written as the sum of terms that arise from the temperature change D T , the volume-fraction change Df , and the light intensity I r , t on the sample.

So, Z-Scan experiments with different time-scale square waves can be used to study these different processes. The experimental results obtained until now with the FRS and ZS techniques can be summarized as follows:. SFF with particles coated with nonionic surfactants dispersed in nonpolar fluid carriers behave as SFF with particles coated with cationic surfactants;. In the case of the SFF e. These results still lack a comprehensive theoretical picture and, probably, different mechanisms take place in the thermodiffusive behavior of these complex fluids [39].

Bringuier and Bourdon [42] prosed a kinetic theory, based on the analysis of a Brownian motion in a nonuniform temperature profile, in order to predict both signs of the Soret coefficient. A distinguishing feature of the research area in ferrofluids is the ample applicability of these materials. A big effort was made by chemists and physicists during a good part of last century to synthesize stable magnetic fluids, motivated by the perspective of many and important technological uses.

Although non-stable suspensions of magnetic particles in liquids have been produced much earlier, the first synthesis of a ferrofluid was reported in the pioneering work by Papell [1], in After this, an increasing scientific production took place in the area.

A remarkable feature of this field is that the number of patents is about one half the number of papers, a clear confirmation to the perspective of ample applicability of ferrofluids. The research field of magnetic fluids is a multi-disciplinary area: Chemists study their synthesis and produce the ferrofluids, Physicists study their physical properties and propose theories which explain them, Engineers study their applicability and use them in technological products, Biologists and Physicians study their biomedical possibilities and use them in Medicine and in research on the biological area.

These properties make the magnetic fluids useful for many technological, biological and medical purposes, as well as a help in materials science and engineering research. In the following sub-sections we comment on some of these applications. Of the many technological applications of magnetic fluids we will single out four main categories: a Dynamic sealing; b Heat dissipation; c Damping; d Doping of technological materials.

In many equipments there are two or more different ambients, which have to be hermetically isolated from each other but some shaft has to carry energy rotation from one ambient into the other. For example, a motor has to be in an open place, where it can be cooled down by the ambient air or some refrigeration mechanism, while a shaft has to go from it into an absolutely clean place, where it has to rotate something.

This is the case, for example, of the hard disks of computers, which have to operate in an hermetically closed box because any grain of powder or even smoke may spoil the reading and writing process. Therefore it is necessary to seal hermetically the hole through which the axle passes. This is achieved by making the hole inside a magnet see Fig.

A groove in the shaft is filled up with ferrofluid, which is kept in place by the magnetic field, obstructing the passage of any impurity, but leaving the axle free to rotate, because the obstructing material is liquid. One way of extracting heat from an equipment which heats up by functioning, and so keeping it not too hot, is by using a good heat conductor which connects the equipment to some mass which has much bigger heat capacity and, perhaps, much bigger open surface to dissipate heat.

In some cases the good heat conductor must not be a solid, because it would block the equipment's operation for example, if it has to vibrate. One way to achieve the desired goal is by using a ferrofluid as heat conductor. A non magnetic liquid would flow away from the place where it is supposed to operate. A good example is a loudspeaker, whose coil heats up by functioning and the ferrofluid is kept in place by the magnetic field of the magnet which is fixed on the loudspeaker's horn.

Nowadays most of the high power loudspeakers are equipped with ferrofluid. Finally, I only finished like 30 minutes ago, so I hope it will all look good tomorrow. If something bad happens I will make another post. You will get a lot more ferrofluid for your money if you buy just the ferrofluid, however making the toy is a bit of work. I don't have a full answer, but a hint at how to choose your liquid. You want the ferrofluid not to mix with it, so your fluid has to be immiscible with the ferrofluid solvent or carrier fluid; ferrofluids are colloidal suspensions.

Secondly, you want the ferrofluid solvent not to touch the glass, to avoid staining. In order to do so, you need your liquid to wet the bottle more than the mineral oil, i.

You can play on both the bottle material and nature of the fluid to achieve this. I suggest protic polar solvents, which should wet regular glass well: water, ethanol, isopropanol, acetic acid, …. No biggie. For aqueous ferrofluid, silanize the glass with Rain-X automotive windshield treatment.

If you want both a hydrophobic and oleophobic glass surface, fluoroalkylsilane surface coupling. What about an ammonia-based fluid mix? I saw one guy on youtube that said he made it with Windex the liquid was a clear greenish color and another guy said he made it with Windolene he's in the UK but it went cloudy after a while.

Meanwhile doing some other research I found someone who says their product they sell ferrofluid desk toys should be kept at room temp and will go cloudy if kept too cold. My guess is the guy in the UK had it accidentally get too cold.. Sign up to join this community.

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