Depending on the size & material f the nanoparticles, they may align to the earth magnetic field :)

Yes, some type of aligned structures can be observed even in the absence of magnetic field during the evaporation of solvent.  The alignment may not be a chain formation always.  Such alignments are known as self assembly of the structures.  This may be the collective effect of drag forces due to solvent during evaporation or Brownian energy or short range van der Waals interactions, adhesive forces between the substrate and particles, etc. The research of self assembled structures is very much interesting and little unexplored yet. 

Depending on the size & material f the nanoparticles, they may align to the earth magnetic field :)

Chain formation and magnetic alignment of nanoparticles happens in some bacteria, where clearly, the particles are in some kind of solute phase and subject to thermal agitation. This, however (as far as I am aware of) concerns the formation of a single chain of particles, and I do not know what kind of infrastructure these bacteria may provide to promote chain formation.

In a macroscopic solution I could also think that chain formation may occur, but I would also expect them to be not totally straight but subject to fluctuations. Also, different chains will in generally not be aligned with respect to each other unless that is being favored by some external field (and this could be the earth's magnetic field).

Whether or not magnetic alignment will occur shall depend on the dipolar coupling strength compared to thermal energy. Larger coupling requires larger moments, which in turn require larger particles (for a given material). Larger particles will lead to larger distances between their centres and hence a decrease in dipolar interaction. You will have to figure out the corresponding energy terms. 

Years ago I had a glimpse into a book by M. Pileni on magnetic particles. Aligned films where discussed in there and that might be a useful source of further information.

Magnetotactic bacteria (gryphowaldensis spirillum  etc.) use so called "magnetosomes" (i.e. an array of ironoxide nanoparticles) to navigae in teh earth magnetic field. There are north-seeiking as well as south-seeking bacteria, depending on the hemisphere.  If magnetic align themselves upon evaporation is likely, but to make sure that you get what you are seeking for, just place a permanent magnet close (or under ) the beaker so that you always get the same alignment (if that is what you are looking for).

exactly the kind of bacteria I was referring to...

Yes, magnetic particles in a colloid can be agglomerated or form short chains (dimers / trimers etc.) in the absence of an external magnetic field. Formation of the zero-field assemblies in a ferrolfuid is mainly dependent on the size, material of the particles, concentration of the particles as well as the coating layer.  In a colloidal suspension, the particles should be coated by an appropriate steric layer to prevent particles approaching each other. The concentration of the suspension is important too because the effectiveness of the dipole-dipole energy depends on the distance between the dipoles. In brief, for the particles to escape agglomeration, thermal energy (kBT) should overcome dipole-dipole energy (mu0M2.V/12). In the absence of a magnetic field, there are two important stability requirements  for ferrofluids: stability  against magnetic agglomeration and stability against gravitational field. 

In the process of evaporation,  Brownian motion (thermal energy) will become less effective and dipole-dipole energy, instead, will become dominant that results in the formation of dipolar chains in the sample. 

A comprehensive theoretical review of the above can be found in "Effective heat capacity of ferrofluids – Analytical approach" and "Direct observation of field-induced assemblies in magnetite ferrofluids"

http://www.sciencedirect.com/science/article/pii/S1290072914001288

http://scitation.aip.org/content/aip/journal/jap/117/10/10.1063/1.4914484

Probably not, depending on what size they are. They could be so small they are superparamagnetic (thermal idiots). With no external magnetic field they have zero remanence. (answer  in collaboration with material scientist Dr. A. Hadad)

I believe that it is possible. But it will depend on the values of the magnetization and magnetic anisotropy (including shape anisotropy) of the particles, blocking temperature and concentration of particles in solution. In addition, the influence may have stray magnetic fields (background) or stray field of the substrate used.

I do agree with Kinnari  Parekh. Without field, the distribution will be very random. As I have recently tried this experiment and I looked with bigger field of view in optical profiler, but without field it is not possible to get. I think it may depend on the moment of the particle itself. It may be possible if you try to do it over a magnetic thin film instead of  Si or any non-magnetic substrate.

Thanks to all of you for your opinions and answers. I would like to confirm that the used magnetic nanoparticles are of average size 5 nm and hence their inherent dipolar moment might not be responsible for any chain formation. It should be mentioned that the system I obtained was not compeletly composed of  nanoparticles chains, but, some network structures, where chains of short lengths are aggregated in random diractions. Therefore, I also  think the role of possible short range interaction like vanderwaal force can be reason for obatining such assembled strucure during solvent evaporation as mentioned by Kinnari Parekh and  N.S.Susan Mousavi.  

Yes, I do agree. On my thesis research, I investigated the influence of magnetostatic interaction. There, when magnetic (cobalt) nanoparticles were deposited on substrates, in some of the samples we observed self assembly of clusters in a ring pattern, though the substrates were non-templated. It is attributed to dipolar interaction. As Kai and others are noted it above, long-range order (pattern) formation also depends on other factors such as the relative strength of the magnetocrystalline anisotropy over the dipolar interaction strength, and even on the kind of the crystalline anisotropy (weather uniaxial or cubic).  Also, in micromagnetic simulation, flux-closure state is common to observe in a randomly distributed nanoparticles with cubic crystalline anisotropy.