Surfactants play a key role in stabilizing nanofluids, helping prevent the nanoparticles from clumping together or settling. This stabilization not only keeps the fluid's properties uniform but also tends to increase the fluid's viscosity. Essentially, when you add surfactants to nanofluids, they coat the nanoparticles and can form structures like micelles, increasing the overall resistance to flow within the fluid. So, as you increase the concentration of surfactants, the viscosity typically goes up. This happens because more surfactant means more coating and structuring around the nanoparticles, making the fluid thicker. The exact increase in viscosity can vary depending on the type of surfactant used, the characteristics of the nanoparticles, and the nature of the base fluid, but the general trend is that more surfactants lead to higher viscosity.

When you add surfactants to nanofluids, they help spread out the nanoparticles evenly through the fluid. This is great for keeping everything stable, but it also means there are effectively more particles floating around, which thickens the fluid. As you increase the amount of surfactant, you might start to see micelles forming. These are like tiny clusters or bubbles that trap several nanoparticles or molecules inside, which adds to the fluid's thickness.

Surfactants can also wrap around each nanoparticle, creating a sort of layer that adds resistance when the fluid moves—think of it as trying to swim with clothes on versus just a swimsuit. The more surfactant you have, the stronger these interactions become, which again makes the fluid thicker.

Plus, when there's a lot of surfactant, it can change how the molecules interact with each other and with the base fluid, leading to increased stickiness between molecules. At high enough concentrations, the fluid can even start behaving a bit like elastic, stretching and bouncing back, which definitely adds to the viscosity. So, in a nutshell, more surfactants mean more structures and interactions happening in the fluid, which all contribute to making it thicker and slower to flow.

For eg. In a cooling system using a water-based nanofluid with copper nanoparticles, adding a surfactant like Sodium Dodecyl Sulfate (SDS) helps keep the nanoparticles evenly dispersed, preventing clumping. While SDS effectively stabilizes the nanofluid, adding too much can increase the fluid’s viscosity, making it thicker and harder to pump. It's essential to balance the surfactant concentration to maintain fluid efficiency without excessive energy costs.

Depends on phases and HLB.Normally fluid viscosity increase with concentration of surfactants.

Hi, it depends on what happens with the system. Usually, the surfactants are added to stabilize separated phases by forming an adsorbed brush (for suspensions) or an interfacial layer (for emulsions), as Neela Gayathri Ganesan described. In a two-phase system, the dispersed phase represents an obstacle to the flow of the continuum phase, which must flow around it, making the real flow path longer while the macroscopic flow appears slower. That is perceived as increased viscosity by the very definition of viscosity as a shear stress over shear rate.

The adsorbed surfactants increase hydrodynamic volume, forming a larger obstacle to flow around and contributing to higher viscosity. However, a specific combination of parameters can give rise to some non-trivial cases. See the papers below for more details.

Article Bulk Polymer Nanocomposites with Preparation Protocol Govern...

Article Super- and Sub-Einstein Intrinsic Viscosities of Spherical N...

Surfactants in high concentration can form mesophases of packed micellar structures. The accumulation of micelles would result in too many interactions, so that they need to reorganize and get packed. Packing increases friction to move in the liquid media, and this increases the viscosity and even gel-like dispersions can be formed, also known as liquid crystals.

The molecular structure of the surfactant influences greatly on this, because some surfactant molecules can be packed better than others. For example, two-tailed surfactants tend more to form packed structures. Surfactants with a critical packing parameter closer to 1 will tend to get easier packed.