Ny teknik fungerer som accelerator, bremse til mikroskopiske dråber

En lille vanddråbe er på vej, tager fart, mens den glider langs en tynd strækning, fladt terræn. brat, den rammer et groft område - det mikroskopiske svar til glasfartbump, hvori dråben sætter sig og stopper død.

Dråben ser ud til at være parkeret, forankret på plads. Men i modsætning til deres makro-modstykker, disse mini fartbump er let at flade. Stephen Morin ville vide; han overvågede deres konstruktion. Så kemikeren ved University of Nebraska-Lincoln fortsætter med at strække det elastiske materiale, som de sidder på, glatte vejen, og så går dråben af ​​igen, piler hen over den perfekt vandrette overflade.

Stop-and-go-bedriften er blot en af ​​flere, som Morin Group har afsløret via sit seneste ægteskab af kemi og elastiske polymerer. Frugterne af det ægteskab? Hidtil uset kontrol over transporten af ​​mikroskopiske dråber, potentielt give nye tilgange til selvrensende materialer, vandhøstningsteknikker og andre, mere sofistikerede teknologier.

Centralt for holdets tilgang er begrebet fugtbarhed – uanset om en dråbe perler op eller spreder sig ud på en overflade, afsløre den overflade som enten hydrofob eller hydrofil, henholdsvis. Inspireret af noget banebrydende forskning fra begyndelsen af ​​1990'erne, Morin og hans laboratorium begyndte at skabe befugtningsgradienter:overflader dækket af små kemiske "ramper", der gør dem hydrofobe i den ene ende, men hydrofile i den anden.

"Det viser sig, at hvis man har sådan et kemisk mønster, når du placerer en dråbe i den hydrofobe ende, denne befugtningsgradient vil drive dråben spontant til den hydrofile side, " sagde Morin, lektor i kemi i Nebraska.

Selvom et interessant fænomen i sig selv, Morin og ph.d.-kandidat Ali Mazaltarim ønskede at se, om de kunne tilpasse den passive transport til en aktiv, dynamisk proces, der bedre egner sig til applikationer. De henvendte sig til den slags elastiske materialer, som Morins team har belagt med kemiske mønstre siden 2015, om man skal lave overflader, der kun reflekterer lys, når de strækkes, eller filtrere partikler baseret på form.

Som det ofte havde gjort tidligere, holdet startede med en blød, smidig silikonefilm. Forskerne strakte den film, før de behandlede den med ultraviolet ozon for at producere et mikroskopisk tyndt lag silica, den primære komponent i det meste glas. De beklædte derefter visse dele af silicaen med tætte krat af vandafvisende molekyler; andre sektioner blev stort set eller helt nøgne, skabe en befugtningsgradient, der kunne drive dråber fra det hydrofobe til det hydrofile.

At få en vis kontrol i realtid over bevægelsen af ​​disse dråber var så et simpelt og bogstaveligt spørgsmål om at give slip. Afslappende den forstrakte silikonefilm introducerede rynker i silicaen, svarende til hvordan et plaster placeret over albuen på en bøjet arm vil rynke, når armen rettes ud. Morins team havde mistanke om, at disse rynker kunne introducere tilstrækkelig ruhed til at bremse dråbernes hastighed, selv på de hydrofobe strækninger af overfladen.

Eksperimenter bekræftede hypotesen:I deres fuldstændig afslappede, rynket tilstand, de hydrofobe strækninger kunne stoppe dråberne alle sammen; i deres helt stramme, glat tilstand, de fragtede dråberne videre, som de plejer.

Forskerne har siden finpudset denne kontrol ved at strække og slappe af filmene for at starte og stoppe dråberne sekund for sekund. They've even shown the ability to challenge gravity, transporting droplets up inclines steeper than those reported in prior research.

Rough riders

Whether, og hvor hurtigt, a droplet will move depends in part on the severity of a wettability gradient. When the transition from hydrophobic to hydrophilic occurs over a short distance, the droplets speed across the surface; when that transition stretches over a longer distance, the droplets lumber at a slower pace. The "steeper" the gradient, med andre ord, the greater the driving force and velocity of the droplets. Other factors, including droplet size, are well-known contributors, også.

But the team was also finding that its acceleration and braking systems depended not just on the presence of the microscopic speed bumps, but also their height and spacing, both of which seemed to be influencing droplet velocity. From a mathematical and theoretical standpoint, the team realized, the roughness of the surface wasn't getting its due.

To better understand and predict how roughness was affecting droplet transport, Morin and Mazaltarim incorporated the variable into a couple of equations that are traditionally used to quantify the phenomenon. After some tweaking and experimental verification, their resulting model predicted the specific roughness needed to slow or stop a droplet of any given size—along with the minimum size needed to overcome that roughness and other factors that resist a droplet's movement.

At, på tur, allowed the team to craft surfaces that would transport larger droplets while leaving smaller ones in place, or trigger the departure of the latter only when stretching the elastic film beyond a certain threshold. And that, the team said, could prove useful in sorting different liquids for analytical or other purposes.

The ability of such a simple technique to yield such precise, predictable behavior makes it promising for a range of other applications, Morin said. The team has already illustrated its potential in self-cleaning materials by dirtying an elastic surface with metal dust, then stretching it to trigger a cascade of droplets that carried away all dust in their path. The harvesting of water for urban agriculture, livestock or potable water might benefit from a similar approach.

"You could imagine fabrics where you collect droplets at one section, " Morin said, "and then you actuate the surface, which then drives them to some sort of a storage container."

There's also the possibility of expanding on the functionality of materials that are designed to remove sweat from skin or droplets from other surfaces. The latter could potentially help cool energy-generating systems that produce sizable amounts of heat.

"A lot of research in that area focuses on hydrophobic and superhydrophobic surfaces that have unique heat-exchange properties, " Morin said. "One could use the evaporative cooling effect of sweat as inspiration. But we imagine a more active system, where you're literally using a droplet to collect heat and then actively moving it somewhere else to remove that heat.

"That's a good thing if you're actively trying to cool any sort of a device. This just presents a new way of achieving that type of outcome."

Further down the line, Morin sees promise for calibrating the technique to transport droplets in two dimensions rather than just one. Managing that, han sagde, could make it a viable alternative in so-called lab-on-a-chip technologies that direct, mix and then analyze microscopic samples of liquids.

"We have the ability to really dial in the properties of the gradients and how they couple to the micro-texture of the surface, " Morin said. "So I think there's a lot of leeway in terms of how you design the system to get a specific performance outcome."

The team reported its findings in the journal Naturkommunikation .