All's well that gels well

A simple solution to potentially toxic lubricants is around the corner. An eco-friendly lubricant can be developed with common salt, water and tiny silica particles, abundantly found in beach sand, says a team from the Indian Institute of Technology (IIT) Kanpur. 

Oil- and grease-based lubricants are widely used in a wide array of applications, including those in industry and automobiles. These, however, are not environmentally friendly as they are derived from mineral oils. Besides, they use a range of toxic additives. 

The global lubricant market was pegged at $175 billion in 2024 and is projected to reach $271 billion by 2034. The Indian lubricant market, valued at ₹35,000 crore in 2024, is expected to grow at a compound annual growth rate of 4.4% till 2030. 

The IIT Kanpur team, led by Manjesh Kumar Singh, Associate Professor of Mechanical Engineering, and Yogesh M. Joshi, Professor of Chemical Engineering, has developed a water-based gel-like lubricant that has silica particles and common salt as ingredients. The scientists recently published the work in the Journal of Colloid and Interface Science (bit.ly/Lubricant-Manjesh). 

Building on the existing knowledge that silicon nanoparticles have good lubrication properties and that they form a gel in the presence of salts such as sodium chloride, the researchers decided to explore the gel's lubrication and anti-wear properties. For the study, they used colloidal silica, which has suspensions of fine, amorphous, nonporous and spherical nanoparticles with diameters in the range of 20 to 22 nanometres (one nanometre is one-billionth of a metre). This colloidal silica solution was mixed with an aqueous solution containing common salt and shaken vigorously for five minutes. When the mixture was left undisturbed for 48 hours, a gel was formed.

Subsequent experiments using this nanoparticle gel showed that it had a combination of super-low friction and negligible wear. It reduced the friction coefficient by 97% on both dry and wet surfaces.

The gel belongs to a class of lubricants with thixotropic properties — a substance which remains as a gel when no pressure is applied but transforms into a solution under pressure. Toothpaste is one such thixotropic material. 

"This is the first such study that explored lubrication and anti-wear properties of such a silica gel," says Singh, a co-corresponding author of the paper. The scientists who tried varying concentrations of silica and common salt found that the gel containing 13% silica nanoparticles and 7% salt had optimal properties. 

"Because of its thixotropic properties, this new lubricant can be applied on vertical contacts, unlike oil-based lubricants that may fall off (such surfaces)," says Singh. A similar study by the team published last year (bit.ly/clay-manjesh) showed that clay nanoparticles, too, formed a gel when mixed with a common salt solution. "The clay-based gels offer good anti-wear properties but are not good for lubrication," he says. 

The nano-silica-based colloidal gel was the next step in their work and was found to be very promising. The team now plans to develop plant-based additives that can help retain and even enhance the lubricant and anti-wear properties of the gel in high-pressure and high-temperature conditions. "Whenever we design lubricants, we have to add additives which are required for a smooth functioning of the lubricant in extreme pressure and extreme temperature conditions. We want to focus on bio-friendly substances for them as well, leading to a totally eco-friendly lubricant. This is our ultimate goal," says Singh. 

As the lubricant is water-based, there was concern about corrosion. The current tests were carried out on discs made of austenitic stainless steel 304, commonly used in household items and generally not corroded by water. The scientists are hopeful that they will be able to identify plant-based extracts that can stymie corrosion on other surfaces as well. 

Singh says their study highlights the potential of the water-based thixotropic colloidal nano-silica gel's lubrication properties for metallic surfaces. It also shows that the lubricant's efficacy can be further fine-tuned by optimising the gel's rheological properties, such as viscosity, gel strength, ageing dynamics, and yielding behaviour. 

"The work certainly looks interesting," says Nitya Nand Gosvami, Associate Professor in IIT Delhi's Materials Science and Engineering Department. He thinks the gel lubricant may find applications in industrial sectors where water-based lubricants are used.