I don't think the statement 2 is true. On the contrary, the stability of NPs colloids can improve with increasing temperature.

thanks MR. Prodanov

But in some article it is reported that stronger Brownian motion leads to increase the amount of aggregation.for example :

Aggregate fractal dimensions and thermal conduction in nanofluids

1 - Brownian motion is driven by thermal fluctuations of the solvent. Increasing the temperature increases the fluctuations and hence the rate of the Brownian random walk process.

2- When the Brownian motion is faster then the nano particles explore the space faster and are therefore more likely to collide (this assumes lots of other things are constant, which may in reality not be).

3 - I assume the fractal dimension of the aggregates is less than 3 such that you form conductive particle chains that span the space?

Hi Hamid! We're carrying out experiments and modeling of nanofluids (and nanoemulsions). Increased Brownian motion is a trivial consequence of increasing T, because (for dilute systems) D = k_BT/(const.* \eta}, i.e. the diffusion coefficient grows linearly with T (and is inversely proportional to the solvent viscocity \eta).

Whether or not the particles aggregate depends on the zeta-potential, which measures the interparticle interaction strength. If the interaction is repulsive enough, there should be no aggregation, but this has to be tuned by e.g. changing the ph. We have a recent modeling paper on colloidal aggregation published.

What happens to thermal conductivity depends on the system. If the particles have a lower thermal conductivity that the solvent, increasing volume fraction will cause the conductivity to go down (according to the Einstein relation for dilute systems). I would imagine that large aggregates enhance this effect.

Dear Hamid,

I'd like to add some points about your question from a brief literature review point of view. A number of possible mechanisms have been proposed for the enhancement of effective thermal conductivity, which included Brownian motion effect, interfacial liquid layering, clustering and networking. We are facing 2 different ways: For nanofluids application under forced convective conditions, possible contributions to the complex behavior of the nanofluids may include the Brownian motion, particle rotation and its associated micro convection, particle migration and the resulting non-uniform property profile, and suppression or interruption of the boundary layer. The mechanisms associated with nanofluids under natural convective heat transfer conditions are expected to be quite different including particle-fluid slip and sedimentation of nanoparticles.

For more info about your no. 3 question, please look at our publication (http://dx.doi.org/10.1007/s10973-014-4048-0)

Good luck

Hi,Hamid,for your second question,when the Brownian motion of nanoparticles turn stronger,the collision frequence of nanoparticles will increase,and nanopartilces are more likely to collide with each other.However,the stronger collision may lead to devision of the aggregated clusters.Whether stroger Brownian motion result in increase of amount of aggregation depends on the interactions like attraction and repulsion between nanoparticles.

For your third one, I noticed publications revealed that aggregation of nanoparticles could increase thermal conductivity of nanofluids, and some demonstrated thernrmal conductivity of nanofluids ismore higher at smaller scale nanoparticles. So it is still a controversial question.

Hi Hamid san,

The heat aggregation studies by dynamic laser light scattering show that even the aggregation of nano paricles in the fluidic medium depends on the medium chemical properties such as pH and also vary from material to material. However, from the physical point of view heated nanoparticles may experience stronger Brownian motion due stronger thermal and local density fluctuations. 

But a relation between stronger Brownian motion with increased aggregation need to be confirmed theoretically as well as experimentally.

Hi Hamid,

About your 2nd question " Stronger Brownian motion leads to increase the amount of aggregation  ".  It is not true always. The particles aggregate depends on the zeta-potential. If the Zeta potential of the system is high , then possibility of forming aggregates is very less.

About your 3rd question. In general the nature of the aggregates and percolating conduction path dictates the thermal conductivity of nanofluids. As far as the aggregates are well dispersed , the thermal conductivity will increase. in some case with time the aggregates tend to come closer due to strong interaction between them results in the formation of big clusters, which eventually settle down due to gravitational forces. In such case the thermal conductivity decreases with time.

Hi Hamid,

The stability of nanofluids is dependent on a combination of van der Waals and electrocstatic force interactions. The former is related to the material properties of both particles and their suspended media and the latter is related to the particle zeta potential and the Debye screening length. It turns out that as increasing temperature particles likely get more aggregation.

Our new, careful heat conductivity measurements reveal that there's no significant increase of the conductivity in the case of of nanoemulsions [S. Saarinen et al., Turbulent heat transfer characteristics and thermal properties of n-decane-in-water nanoemulsion fluids and micelles-in-water fluids, to appear in Int. J. Mass and Heat Transfer (2014)]. In fact, for the nanoemulsions here, the conductivity decreases with increasing volume fraction according to the Einstein equation (the emulsion phase has a lower thermal conductivity that that of water). Thus, any reported "anomalous" heat conduction behavior should be taken with caution due to aggregation and other possible ill-controlled physical effects.

The interesting thing is that, in forced heat convection, there's an enhancement, but only in the turbulent regime, where Re > 7000. There's no enhancement in the laminar regime for any accessible particle sizes (< 10 nm) or concentrations. Our results also conclusively rule out particle clustering or liquid layering as sources of heat transfer enhancement.

hi,

1. Yes it has been shown in numerous articles that BM has direct relation with temperature.

2. it has been reported that increasing BM will results in increased aggregation of NPs as it increases their collision rate, but it's not a generally accepted fact. 

3. aggregates could increase or decrease the thermal conductivity of NF. it depends on the size of aggregates and their volume fraction, as increasing the size of cluster will result in decreased motion and consequently decreases the enhancement due to particles motion.