A gecko is sitting at the ceiling of a room, waiting to catch a fly. It is quite surprising that animals like geckos, which have the weight of a chocolate bar, can just hang at walls. When closer investigated, they do not use any sticking substance or suction cups. Their feet are soft without any claws. Recent experiments performed at the CERN CLOUD chamber found that the gecko force, which keeps geckos sticking to walls, is also important on the molecular level, sticking molecules together. The study concludes that this magic gecko force could be one puzzle piece to explain why air pollution is formed from tiny aerosol particles in metropolises around the globe.
In urban areas, power plants, residential heating and traffic lead to higher concentrations of sulfur dioxide. Once emitted into the atmosphere sulfur dioxide is oxidized and forms sulfuric acid. And sulfuric acid is one of the main trace substance responsible for the formation of aerosols. While extremely sticky, sulfuric acid rapidly forms small clusters, when two molecules meet. However, this takes a long time in the atmosphere. The typical concentrations in most urban areas are nowadays quite low as sulfur dioxide emissions were strongly reduced during the last decades. Once two sulfuric acid molecules met, this pair cannot grow as it takes again some time to find a third or a fourth molecule.
But time is crucial for small clusters in urban areas. The air is full of large soot particles emitted from trucks or chimneys. The small clusters move around very fast and quickly adhere to the large particles before finding more small molecules, growing, becoming slower and hence surviving that scavenging. For small clusters it is all about grow or die, as explained earlier. So how can they quickly find other sulfuric acid molecules? The study published in Atmospheric Chemistry and Physics in July, investigated the growth of nanoparticles from sulfuric acid with high precision aerosol instruments down to the smallest particle sizes of about 1 nanometer. Here, the gecko force becomes important. Typically, we think of the collisions of clusters as if two tiny spheres would collide. And if two solid spheres just miss each other – they miss each other.
Sulfuric acid is different. Its molecular structure is different. If another sulfuric acid molecule just passes, they get attracted to each other and finally collide. This attraction is called the van-der-Waals force, which also causes the adherence of millions of hair like lamellae structures on the gecko’s footpad to the wall. The gecko force helps sulfuric acid molecules to find each other, which lead to a more than two times faster growth of the smallest sulfuric acid clusters. This is one possibility how they can grow and survive in cities, becoming also relevant for air pollution. So next time you see a gecko at the wall, keep in mind what physics make this happen!