Nav: Home

New insights into the evaporation patterns of coffee stains

March 09, 2016

Few of us pay attention to the minutiae of coffee stains' deposition patterns. However, physicists have previously explained the increased deposition of ground coffee particles near the edge of an evaporating droplet of liquid. They attributed it to the collective dynamics of ground coffee grains as the liquid evaporates along the contact line between the liquid coffee and the table. This kind of dynamics also governs microchip production, when particles are deposited on a substrate by means of solvent evaporation. However, until recently, explanations of how such evaporation patterns are formed did not account for the effect of the mutual interactions between electrically charged particles. Now, Diego Noguera-Marín from the University of Granada, Spain, and colleagues have found that particle deposition may be controlled by the interplay between the evaporation of the solvent via convection and the previously identified collective diffusion of suspension nanoparticles. These findings appear as part of an EPJ E topical issue, entitled Wetting and Drying: Physics and Pattern Formation.

In this study, the authors set out to pump out the nanoparticle suspension to study why particle deposition is driven to recede at the contact lines between solvent and substrate. Unlike typical prior experiments focusing on free drop evaporation, the sustained evaporation was, in this case, not tied to the motion of the contact line at the macroscopic scale, between solvent and substrate, which directed the formation of the final deposit. Indeed, this approach makes it possible to keep the particle concentration constant throughout the entire experiment.

When the evaporation flow is weak, the authors found, the deposition of nanoparticles can be suppressed. Then, long-range inter-particle repulsion becomes important. As a result, particle transport is mainly governed by diffusion via convection. However, the diffusion-based flow is only relevant at low particle concentrations, where the concentration gradient between the contact line and the bulk of the nanoparticles suspended in the solvent becomes important.
-end-
Reference: Impact of the collective diffusion of charged nanoparticles in the convective/capillary deposition directed by receding contact lines. D. Noguera-Marín, C. Lucía Moraila-Martínez, M. Cabrerizo-Vílchez, and M. Ángel Rodríguez-Valverde (2016), Eur. Phys. J. E 39: 20, DOI 10.1140/epje/i2016-16020-y

Springer

Related Nanoparticles Articles:

Study models new method to accelerate nanoparticles
In a new study, researchers at the University of Illinois and the Missouri University of Science and Technology modeled a method to manipulate nanoparticles as an alternative mode of propulsion for tiny spacecraft that require very small levels of thrust.
Actively swimming gold nanoparticles
Bacteria can actively move towards a nutrient source -- a phenomenon known as chemotaxis -- and they can move collectively in a process known as swarming.
Nanoparticles take a fantastic, magnetic voyage
MIT engineers have designed tiny robots that can help drug-delivery nanoparticles push their way out of the bloodstream and into a tumor or another disease site.
Quantum optical cooling of nanoparticles
One important requirement to see quantum effects is to remove all thermal energy from the particle motion, i.e. to cool it as close as possible to absolute zero temperature.
Nanoparticles help realize 'spintronic' devices
For the first time researchers have demonstrated a new way to perform functions essential to future computation three orders of magnitude faster than current commercial devices.
More Nanoparticles News and Nanoparticles Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Rethinking Anger
Anger is universal and complex: it can be quiet, festering, justified, vengeful, and destructive. This hour, TED speakers explore the many sides of anger, why we need it, and who's allowed to feel it. Guests include psychologists Ryan Martin and Russell Kolts, writer Soraya Chemaly, former talk radio host Lisa Fritsch, and business professor Dan Moshavi.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...