| Something as simple as the motion of water drops on surfaces should actually be understood – one would think. In fact there are still numerous unanswered questions about the forces acting on a sliding droplet. | |
| A team of researchers from the Max Planck Institute for Polymer Research in collaboration with colleagues from TU Darmstadt has now discovered: In addition to surface energy and viscous friction within the droplet, electrostatics also play a significant role. | |
| The results were recently published in the journal Nature Physics (“Spontaneous charging affects the motion of sliding drops”). |
| Raindrops hit the car window and the wind pushes the drops to the side. Even today, no one has been able to precisely predict how the drops move on the windshield. Yet such an understanding is important in numerous areas, such as autonomous driving: For example, cameras installed in the windshield are supposed to keep an eye on the road and the traffic situation – for this, the surface of the windshield must be designed in such a way that the drops are completely blown down by the airstream and the view remains clear even in the rain. Other examples with the opposite sign are applications where drops need to stick to surfaces, such as spray paint or pesticides. | |
| “Until now, it was assumed that the surface coating was responsible for how the droplet moves on a surface – that is, the first few molecular layers,” says Prof. Hans-Jürgen Butt, who is director of the “Physics of Interfaces” department at the Max Planck Institute for Polymer Research. | |
| For example, it depends on the surface whether a spherical or a flat droplet shape is formed. If the drop likes the surface, it presses itself flat onto it to make as much contact as possible. If it does not like the surface, as in the case of the well-known lotus effect, it curls up. It was also clear that when a droplet moves, viscous friction – i.e. friction between the individual water molecules – occurs within the droplet, which also influences its movement. | |
Electrostatics cause differences in velocity |
|
| The team of researchers at the MPI for Polymer Research found that neither capillary nor viscoelastic forces can explain the differences in the speed at which droplets move across different surfaces. Questions were raised in particular by the fact that the droplets run at different speeds on different substrates – even if these substrates have an identical surface coating, where no differences would be expected. | |
| The researchers therefore first introduced a mysterious “extra force.” To track it down, Xiaomei Li, a Ph.D. student in Hans-Jürgen Butt’s department, organized a drop race. “I filmed the drops on different substrates, extracted velocity and acceleration profiles from their motion, calculated out the forces that were already known to calculate the force that we had not yet had a look at,” she explains. | |
| The astonishing result: the calculated force agrees with an electrostatic force that the researchers first described in a model a few years ago. “By comparing the experimental results with this numerical model, we can explain previously confusing droplet trajectories,” says Jun.-Prof. Stefan Weber, a group leader in Butt’s department. | |
| If previously neutral droplets slide over an insulator, they can become electrically charged: So electrostatics plays a significant role there. On an electrically conductive substrate, on the other hand, the droplet immediately releases its charge back to the substrate. | |
| “The electrostatic force, which no one had previously considered, therefore has a major influence: it must be taken into account for water, aqueous electrolytes and ethylene glycol on all hydrophobic surfaces tested,” Weber summarizes. | |
| These results will improve the control of droplet motion in many applications ranging from printing to microfluidics or water management to power generation via droplet-based mini-generators. |
News
China makes early progress in using sound waves to stimulate damaged brains
Chinese researchers are testing whether focused ultrasound can help restore consciousness in patients with severe brain injuries, adding to a growing international effort to use sound waves to stimulate deep brain structures without surgery. [...]
Micro-drones fly through the body thanks to ultrasound
At the EPFL, micro-drones have been developed that can be moved without a motor or battery.No motors, no batteries, no electronics on board, just small cavities that convert sound waves into thrust: this is [...]
A Single Enzyme Could Hold the Key to Treating a Devastating Brain Disorder
Blocking an enzyme’s production restored brain growth and healthy behaviors in mice engineered to develop Angelman syndrome, pointing to a possible treatment target. Children with Angelman syndrome begin to show difficulties with thinking and [...]
Scientists Develop a Clever New Way To Kill Cancer Cells From the Inside
Scientists have developed an experimental strategy that exploits a chemical byproduct of cancer cells to produce anticancer drugs directly inside them. Scientists in Japan have developed an experimental method that uses a chemical produced [...]
Our books are available worldwide!
Online Sellers other than Amazon, Routledge, and IOPP Indigo Global Health Care Equivalency in the Age of Nanotechnology, Nanomedicine and Artifcial Intelligence Global Health Care Equivalency In The Age Of Nanotechnology, Nanomedicine And Artificial [...]
After only a short training, people in a coma managed to send targeted signals
Researchers have succeeded in communicating with brain-injured patients who are unconscious. They see enormous potential in this for future treatments. Patients who lie motionless in bed after a severe brain injury and do not [...]
MIT Scientists Reprogram Immune Cells Inside the Body To Destroy Cancer
Natural killer cells reprogrammed inside mice eliminated lymphoma tumors and helped damaged livers heal, though whether this approach could help people with cancer or autoimmune disease remains unknown. To make a cancer treatment from [...]
Melanoma mRNA Vaccine Shows Promise in Preventing Disease Recurrence
Key takeaways: Phase 3 trial finds personalized intismeran plus pembrolizumab may better prevent high-risk melanoma recurrence after surgery The trial enrolled 1,137 patients and builds on phase 2 results showing about half the recurrence [...]
Scientists Discover a Powerful New Weapon Against a Dangerous Gut Infection
The promise of an experimental treatment for C. difficile gut infections lies partly in the bacteria it leaves alive, according to early laboratory and mouse research. Broad-spectrum antibiotics can disrupt the gut’s microbial community, [...]
Your Bones May Hold a Hidden Clue to Blood Cancer Survival
For people with blood cancers called myelodysplastic syndromes, osteoporosis may hold an overlooked clue to survival, a three-year observational study suggests. In blood cancers that develop in the bone marrow, an important clue to [...]
Researchers Discover What Reading for Pleasure May Be Doing to Your Brain
Reading for pleasure may offer surprising benefits for the brain, with research linking the habit to better mental health, stronger cognitive abilities, and a lower risk of dementia. Reading for pleasure may do far [...]
Scientists Discover a Hidden Trigger That Keeps Inflammation Alive
Inflammation persists after proteins detach from immune receptors, revealing a potential target for autoimmune treatments. Since a Nobel Prize-winning discovery in the 1990s identified molecules called Toll-like Receptors as the guardians of the human [...]
A Virus Gives Itself Away the Moment It Attacks
Researchers have found the trigger behind a powerful bacterial defense against viruses. Viruses that prey on bacteria could become an increasingly important tool for treating bacterial infections, particularly as antibiotic resistance makes some infections [...]
Scientists Discover Aspirin May Slash Dementia Risk by 70% – but Only for Certain People
Aspirin was linked to a sharply lower dementia risk in some older adults, and their genes may hold the clue. A Monash University analysis found that older adults with a particular genetic profile had [...]
Inflammation May Awaken Dormant Cancer Cells Years After Treatment
The body’s response to inflammation may disturb cancer cells that have been lying silent for years, giving them an opportunity to grow again. Researchers at the Francis Crick Institute have found that gut inflammation [...]
Where do nanoparticles go once they enter the body?
New international best practice guidance, co-written at ANSTO, sets out how to measure where nanoparticles like titanium dioxide end up in the body and how long they remain. The guidance has been shared by [...]
