Suppose you are using a probe to collect a water sample from one liquid bath and transferring it to another, perhaps to measure the water’s Ph or run some other diagnostic test. If the bath from which you collect the sample contains any bacteria or other contaminants and you remove the probe too quickly, bacteria may become “entrained” in the liquid film coating the probe, contaminating it and rendering it useless. 

In a kind of reverse example, you might be collecting the water sample to measure the bacterial content of the water in the bath. In this case, if you remove the probe too slowly, forming only a very thin liquid film, you may fail to entrain — and thus, capture — the bacteria you are trying to count.

The point in both of these examples, say Alban Sauret and Emilie Dressaire, both assistant professors in the Department of Mechanical Engineering at UC Santa Barbara’s College of Engineering, is that speed of extraction matters. “You really have to be careful with what is called the ‘operating condition,’ which really comes down to the velocity of extraction, in order to entrain what you want and leave behind and what doesn’t interest you,” Dressaire says.

In a paper titled “Capillary Filtering of Particles During Dip Coating,” which appeared last spring in the journal Physical Review Fluids and another titled “Capillary Sorting of Particles by Dip Coating” published in July in Physical Review Applied, the researchers describe a new method to avoid contamination of substrates extracted from a contaminated liquid — whether the contaminant is a tiny bacteria, individual particles (like dust), or clumps of particles — and to control the selective entrainment of particles.

The method has everything to do with capillary action, which enables a liquid to flow in narrow spaces without the assistance of gravity and, sometimes, in opposition to it. Here, the liquid is entrained on the substrate via a meniscus in which particles can remain trapped because the capillary force prevents deformation of the interface, which, in turn, prevents particles that are too large from squeezing into the liquid film and becoming entrained. When the velocity was slow, Sauret and Dressaire  observed a thin film of liquid, free of any contaminants, coating the plate. As the velocity increased, the thickness of the film increased, too, and at a threshold velocity, particles were entrained.

Manufacturing and coating processes are just two areas rife with processes that involve dipping some kind of object — a probe, a wire, a sheet of material — into a bath, removing it, and then taking one of several actions, which may include dipping it into another container, testing the captured liquid, using the dipped object as a completed product, or sending it the next step of a multi-stage manufacturing process. 

“The product has to go from one sample to another without contaminating the whole thing,” says Sauret. Or, if the bath contains a coating, conditions have to be such that no dust or other materials from the bath are entrained, resulting in a finish that is less than perfectly smooth. The team discovered further that a particle or an aggregate of particles can be up to five times the thickness of the liquid film and still become entrained.

Prior to coming to UCSB, Sauret worked in collaboration with Saint Gobain, a multinational corporation focusing on construction and high-performance materials. In industrial settings, controlling the entrainment of particles that can bring unique properties to a substrate is a key issue in terms of both the quality of the coating and the cost associated with defects. 

That experience got Sauret to working on the entrainment problem with Dressaire. “We found that, if you choose your withdrawal velocity carefully, you can passively keep suspended clusters away from the film,” Dressaire says.

In their experiments, the researchers observed and characterized several different entrainment regimes as the velocity of withdrawal from the bath increased: from a pure liquid film (slow withdrawal speed), to a liquid film containing clusters of particles, and, eventually, to individual particles that did not cluster (the fastest withdrawal). The results suggest that, because the capillary filtration is an effective barrier against contamination of substrates withdrawn from a polluted bath, it may be useful in preventing bio-contamination. It was also demonstrated that changing the rate at which the object is withdrawn from the liquid allowed them to sort particles by size.
“Our idea,” adds Dressaire, “was that you might not be able to control the conditions in the bath, but what you can easily control is how quickly you pull out the substrate, the probe, or whatever. You’re fighting with the viscosity of the liquid.”

The project involved developing mathematical models to account for the physics of the entrainment process and then designing experiments to test the theory. “The math gives you the thickness of the film on the plate and the properties of the film at the meniscus,” Dressaire explains. “The part of the equation we really care about is for the meniscus, because that determines how many particles are successfully entrained, and which ones get stuck and kind of loop back into the bath. The location of the particle on the meniscus determines whether or not it gets entrained.” 

That holds for different types of substrates, i.e. for fibers as well as for glass plates. “The idea is that a particle needs to get into that critical region, that meniscus, where the fluid can either go back down or go up,” Sauret notes. “When the particle goes up with the liquid as the substrate is pulled from the bath, it deforms the air-liquid interface to squeeze into the film. The interface resists deformation like a rubber balloon, so if the particle is too big, it won’t make it.” 

“The paper examines various entrainment regimes to determine when a single particle is entrained and when it is not — the “carry” of the entrainment process,” says Sauret. “We’ve shown that you can extract a clean substrate from a dirty bath and can control the entrainment of particles. Our future work is to develop new methods to leverage these capillary forces in developing new coating methods with microparticles, opening new and exciting challenges in material science and industrial processes.”