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Science

New study solves mystery of how soft liquid droplets erode hard surfaces

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A new study led by University of Minnesota Twin Cities researchers shows why liquid droplets have the ability to erode hard surfaces, a discovery that could help engineers design more erosion-resistant materials. The above image shows the impact droplets can make on a granular, sandy surface (left) versus a hard, plaster (right) surface. Credit: Cheng Research Group, University of Minnesota

A first-of-its-kind study led by University of Minnesota Twin Cities researchers reveals why liquid droplets have the ability to erode hard surfaces. The discovery could help engineers design better, more erosion-resistant materials.

Using a newly developed technique, the researchers were able to measure hidden quantities such as the shear stress and pressure created by the impact of liquid droplets on surfaces, a phenomenon that has only ever been studied visually. 

The paper is published in Nature Communications.

Researchers have been studying the impact of droplets for years, from the way raindrops hit the ground to the transmission of pathogens such as COVID-19 in aerosols. It’s common knowledge that slow-dripping water droplets can erode surfaces over time. But why can something seemingly soft and fluid make such a huge impact on hard surfaces?

“There are similar sayings in both eastern and western cultures that ‘Dripping water hollows out stone,'” explained Xiang Cheng, senior author on the paper and an associate professor in the University of Minnesota Department of Chemical Engineering and Materials Science. “Such sayings intend to teach a moral lesson: ‘Be persistent. Even if you’re weak, when you keep doing something continuously, you will make an impact.’ But, when you have something so soft like droplets hitting something so hard like rocks, you can’t help wondering, ‘Why does the drop impact cause any damage at all?’ That question is what motivated our research.”






Watch a video demonstrating in slow motion how a water droplet impacts a sandy surface. Credit: University of Minnesota

In the past, droplet impact has only been analyzed visually using high-speed cameras. The University of Minnesota researchers’ new technique, called high-speed stress microscopy, provides a more quantitative way to study this phenomenon by directly measuring the force, stress, and pressure underneath liquid drops as they hit surfaces.

The researchers found that the force exerted by a droplet actually spreads out with the impacting drop—instead of being concentrated in the center of the droplet—and the speed at which the droplet spreads out exceeds the speed of sound at short times, creating a shock wave across the surface. Each droplet behaves like a small bomb, releasing its impact energy explosively and giving it the force necessary to erode surfaces over time.

Besides paving a new way to study droplet impact, this research could help engineers design more erosion-resistant surfaces for applications that must weather the outdoor elements. Cheng and his lab at the University of Minnesota Twin Cities already plan to expand this research to study how different textures and materials change the amount of force created by liquid droplets.

“For example, we paint the surface of a building or coat wind turbine blades to protect the surfaces,” Cheng said. “But over time, rain droplets could still cause damage via impact. So, our research after this paper is to see if we can reduce the amount of shear stress of droplets, which would allow us to design special surfaces that can mitigate the stress.”

In addition to Cheng, the research team included University of Minnesota chemical engineering Ph.D. student Ting-Pi Sun, University of Santiago, Chile Assistant Professor Leonardo Gordillo and undergraduate students Franco Álvarez-Novoa and Klebbert Andrade, and O’Higgins University, Chile Assistant Professor Pablo Gutiérrez.

Heat conduction is important for droplet dynamics

More information:
Stress distribution and surface shock wave of drop impact, Nature Communications (2022). DOI: 10.1038/s41467-022-29345-x
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University of Minnesota

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New study solves mystery of how soft liquid droplets erode hard surfaces (2022, March 31)
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US

McCarthy warns Democrat-backed HR 1 is Pelosi power grab meant to erode election confidence

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House Minority Leader Kevin McCarthy said Sunday that the Democrat-backed H.B. 1 will destroy election confidence and serves as a power grab for House Speaker Nancy Pelosi, effectively destroying America if the sweeping election reform bill passes next month.

“No, I don’t have the most to lose: The American public have the most to lose because we would lose our freedom,” McCarthy, R-Calif., said on Fox News Channel’s “Sunday Morning Futures.” “When you put a bill into Congress, the majority party reserves the first numbers. This is H.R. 1, so this is most important for Nancy Pelosi to hold on to her power.”

EVERY HOUSE DEMOCRAT SIGNS ON SWEEPING HR 1, GOP ARGUES IT WOULD ‘UNDERMINE’ ELECTION INTEGRITY 

Every House Democrat last week signed onto the sweeping election reform bill H.R. 1 – also known as the For the People Act – saying it will expand voting rights and “clean up corruption” in politics. But Republicans have ripped it as a “federal government takeover” and accuse Democrats of trying to change election rules to benefit themselves.

The bill is expected to be considered on the House floor during the first week of March.

“What we have to do is, people lose their confidence in elections, you’ll break society down and we will lose America as we know it,” McCarthy said, addressing host Maria Bartiromo. “We know that there’s a real question, one of the confidence in the last election.

“Going forward, what you have to do is exactly this: You have to clean up the rolls. …  You have to sue to make sure you clean up the rolls. We’ve got to make sure there’s integrity.”

BIDEN’S $1.9T CORONAVIRUS STIMULUS PACKAGE INCLUDES $1B FOR RACIAL JUSTICE PROVISIONS FOR FARMERS 

McCarthy then took a jab at California Gov. Gavin Newsom, who’s facing a strengthening recall campaign likely to make the ballot this fall. He said the governor would be more likely to verify the identifies of those who sign the recall petitions than those who vote in his state. 

“There’s a recall going on in California. I will guarantee you that Gavin Newsom makes sure that they’re checking the IDs of who signs that recall more than they’re checking the IDs of who’s voting in California,” McCarthy said. “You can’t have a mass ballot mailing to people who don’t even request it when these rolls are so bad.

“I think showing an ID. to get on an airplane. Yeah, I could do that. I can also show an ID to vote because in California, it’s just like what they’re doing in the COVID bill. They now want to give vaccines to immigrants, illegal immigrants who are coming in, before giving them to Americans in San Francisco,” he continued. “If you’re illegal, you can vote in a school board race. They want people to be able to vote who are not Americans. We need to have integrity and accountability and elections that people will trust at the end of the day. And that’s why we have to have reforms and that’s why H.R. 1 should be voted down.”

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McCarthy also claimed that out of President Biden’s $1.9 trillion COVID stimulus package, 91% goes toward non-COVID related agenda items, arguing “socialists are taking back the swamp.”

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US

Prospects of convicting Trump erode as GOP grows vocal against Senate impeachment proceedings

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After Democratic leaders announced they would kick off the process to begin the impeachment trial on Monday, Republicans grew sharply critical about the proceedings — and made clear that they saw virtually no chance that at least 17 Republicans would join with 50 Democrats to convict Trump and also bar him from ever running from office again.

In interviews with more than a dozen GOP senators, the consensus was clear: Most Republicans are likely to acquit Trump, and only a handful are truly at risk of flipping to convict the former President — unless more evidence emerges or the political dynamics within their party dramatically change. Yet Republicans are also signaling that as more time has passed since the riot, some of the emotions of the day have cooled and they’re ready to move on.

“The chances of getting a conviction are virtually nil,” said Sen. Roger Wicker, a Mississippi Republican.

“I don’t know what the vote will be but I think the chance of two-thirds is nil,” said Sen. John Cornyn, a Texas Republican and member of his party’s leadership who called the Democratic push to begin the trial “vindictive.”

“From listening to the dynamic — and everything to this point — it’s going to be tough to get even a handful,” said Sen. Mike Braun, an Indiana Republican, referring to possible GOP defectors. “I think so many are getting confused by the fact that we’re doing this – and everybody has views that it’s kind of a constitutional concern.”

The GOP arguments are now coming into sharper focus, claiming the proceedings are unconstitutional to try a former President and contending that the trial is moving on too short of a timeframe to give due process to Trump, claims that Democrats resoundingly reject. But those arguments, Republicans believe, will allow them a way out of convicting Trump without endorsing his conduct in the run up to the deadly mob that ransacked the Capitol on January 6. And Senate Minority Leader Mitch McConnell is likely to land in the same spot as much of his conference, GOP senators believe, although the Republican leader has said he would listen to the arguments first before deciding how to vote.

Politically, most Republicans are not eager to break ranks and draw the kind of attacks that came the way of the 10 House Republicans who voted to impeach Trump last week for the second time in his presidency, this time on a charge of inciting an insurrection.

“Many view it as a game of shirts and skins,” said one GOP senator, referring to how many of his colleagues view the proceedings as a strictly partisan affair.

For the Democrats, the calculation is also tricky. If they seek a longer trial — even as long as the 21 days of Trump’s first impeachment trial in 2020 — with witnesses, they could satisfy some Republicans who are arguing that the trial must give adequate opportunity for Trump to make his case. Yet, doing so could eat away at the first full month of the Biden presidency, while a shorter trial would alienate some Republicans.

“I’m not for any witch hunts,” said Cornyn, who noted he’d be less likely to convict if it were a short trial with no witnesses. “This needs to be a fair and respectable process because whatever we do, it’s not just about President Trump. This is about setting a new precedent and as you know, once we do things around here and there is a precedent for it, then that’s the rule for the next time this happens.”

Among the most likely GOP defectors are Sens. Mitt Romney of Utah, Susan Collins of Maine, Ben Sasse of Nebraska, Lisa Murkowski of Alaska and Pat Toomey of Pennsylvania. But Republican leaders who monitor their conference closely don’t see much of a chance that the list will swell to 17 senators unless something dramatically changes or more is learned about Trump’s role in stoking the violent mob.
“There’s less than a handful of Republicans in play,” said Sen. Lindsey Graham, the South Carolina Republican who is lobbying his colleagues to stick with Trump or risk “destroying” their party.

And even some who had been viewed as possible swing votes are critical of Democrats for trying to start the trial immediately, rather than abiding by the timeline proposed by McConnell to push off the floor proceedings until later in February.

“It’s very problematic, I would say, for the folks who are bringing this right now from a timing standpoint,” said Senate Minority Whip John Thune, the No. 2 Republican, who has been critical of Trump’s conduct and also is up for reelection in 2022. “I think it’s going to be very important whether or not there’s due process.”

Added Murkowski: “I think what McConnell laid down was eminently reasonable, in terms of making sure that we got process. Got to have process and the process has to be fair.”

Collins, the Maine Republican who has been sharply critical of Trump’s conduct, said that she is consulting with “constitutional scholars” about the proceedings. Asked about the GOP senators’ assessment that Trump almost certainly won’t get convicted, she said: “That’s not an unreasonable conclusion, but I just don’t know.”

McConnell himself has privately viewed Trump’s handling of the riots with disdain and has told people they amounted to at least an impeachable offense, while even saying the mob was “provoked” by the former President. But McConnell has made clear to his colleagues that he is undecided — and several Republicans told CNN this week that he could be at risk of losing his perch atop the Senate GOP conference if he votes to convict Trump.

And in the last two days, McConnell has publicly made the case to give Trump’s team more time to prepare. With much of the GOP conference now lining up against conviction, Republicans speculate that the GOP leader will likely vote to acquit as well.

One of the key hurdles the House Democratic managers will have with Republicans is convincing them that a trial is constitutional, as a group of Senate Republicans have argued in recent days that a trial for an ex-president who is now a private citizen is unconstitutional. Such an argument could give Republicans a reason for voting to acquit Trump without addressing his conduct surrounding the insurrection at the Capitol earlier this month.

“I think it’s obvious that the post-presidential impeachment has never occurred in the history of the country for a reason, that it’s unconstitutional, that it sets a bad precedent for the presidency and it continues to divide the nation,” Graham said Friday.

It’s a debate that enters into unprecedented territory, as the Senate has never held an impeachment trial for a President who has left office because such a scenario never arose. But Democrats have pointed to legal scholars on both ends of the political spectrum who say a trial is constitutional. Legal analysts say there’s precedent for a Senate impeachment trial of a former official, as the Senate tried Secretary of War William Belknap in 1876 after he resigned just before the House voted to impeach him.

“It makes no sense whatsoever that a president — or any official — could commit a heinous crime against our country and then be permitted to resign so as to avoid accountability and a vote to disbar them from future office,” Schumer said Friday.

Republican defenders of Trump push back.

Sen. Josh Hawley, a Missouri Republican who has generated blowback for joining with House Republicans to try to overturn Pennsylvania’s election results, distanced himself from Trump’s remarks at the January 6 rally where he urged his supporters to go to the Capitol that day, calling them “inflammatory” and “irresponsible.”

But when asked how they should hold Trump accountable, Hawley said: “Breaking the Constitution and using an unconstitutional process is not the way to do it.”

CNN’s Ali Zaslav, Ali Main and Olanma Mang contributed to this report.

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