Dear Dr. Aou:

at first: "volume or mass" - it of course depends on the nature (polymer or inorganic) of nanoparticles. If to speak about inorganic nanos - one of the leader in this (production)  field is Nissan Chemicals Japan: they produce nano-colloids in water ("Snowtex";

in DMAC, NNP : - they call such products "organo silica soles" (organo because the colloid is stabilized by surfactants; Aluminosol... etc.  Usually the produced by them nanoparticles have dimensions from 10-15 - up to 50, 70, 100nm and are uniquely uniform in size. Well, as I remember - the "dry-weight-portion of nanoparticles could be up to 20w.%

Best Regards,                                    Leonid 

for magnetite nanoparticles max concentration is 25 %   ----- 25 gr /100ml  .please,refer http://www.physiologyweb.com/calculators/percent_solutions_calculator.html

Dear Dr. Aou:

See, most of professional nanofluid satisfy the Maxwell conditions being diluted in essence.

Selection of fluids depending on the application that can be very broad.  Some applications are focused in the nanoparticle (medical biological and medical…etc), another depending on the further interaction fluid and nanoparticle (mechanical seals, cooler fluids, fuel additives, markers…etc). Fluid selected can be polar or apolar. Typically, density and viscosity show further influence on the overall properties.

Selection of nanoparticles depending on the size, chemical composition, crystalline structure, surface defect and its concentration , as well as nanoparticle geometry. Nanoparticle can be metallic (metal and alloys), metalloid, oxide, and semiconductor. Nanoparticle profile can affect nanofluid stability performance. Charge density on the nanoparticle surface is a relevant parameter at nanoparticle oxides and semiconductors, in nanoparticle metal the existence of Plasmon resonance phenomenon should be considered.

Selection of the nanoparticle concentration for a classical nanofluid depending on the base fluid, application and performance. A concentration of particle can be given in mass percent or volume percent. In the practice, both parameters should be considered simultaneously. In the first case, quantities and relative quantities can be determined in mol. In the second case, volumes percent allow theoretical investigations since most of theoretical model at about a wide set of properties models make use volume of nanoparticles as variable.

Selection of the nanoparticle concentration for non-classical nanofluid or highly concentrated nanofluid. Here, the definition of nanofluid is important, then if a nanofluid can be defined as a dispersion of nanoparticles in a proper fluid below of the percolation limit it’s seems feasible. From this point, some important parameter in functional nanofluid emerges. This parameter is nanoparticle stability. Typically, an organic molecule is added to fluid nanofluid to provide stability to nanoparticle against sedimentation.

Selection of stabilizing agent and amount to be used at a functional nanofluid depending on the nanoparticle surface, nanoparticle geometry and nanoparticle concentration. In a broad sense, this agent stabilizing actuate on the nanoparticle surface. Unfortunately, seems that major parts of relevant information on the topic are specific of developers. But, some rules can be highlighted. At least one surfactant molecule (stabilizing agent) should be selected. At least one co-surfactant molecule (co-stabilizing agent) should be selected. From this point, another molecule can be added in similar way to a high performance gasoline, such as: anti-oxidizing, anti-fungi, …In this sense, the limit of nanoparticle charge should be further determined after final development answering the basic question: further characteristic of fluid was preserved?

Best regards

Marcos Nobre

For fluid suspensions you must specify the expected sedimentation rate. The maximum concentration depends on a wide range of parameters starting with density and size and ending with intermolecular interactions (and electric if there are ions).

There is no theoretical upper limit, except for the closest packing volume. If you want to know the practical limits look at the concentrations at which commercial products are sold. No-one want to ship excess water hence those solutions will be as concentrated as possible.