French fries are delicious. French fries with ketchup are a little slice of heaven. The problem is it's basically impossible to pour the exactly right amount. We're so used to pouring ketchup that we don't realize how weird its behavior is. Imagine a ketchup bottle filled with a straight up solid like steel. No amount of shaking would ever get the steel out. Now imagine that same bottle full of a liquid like water. That would pour like a dream. Ketchup, though, can't seem to make up its mind. Is it is a solid? Or a liquid? The answer is, it depends. The world's most common fluids like water, oils and alcohols respond to force linearly. If you push on them twice as hard, they move twice as fast. Sir Isaac Newton, of apple fame, first proposed this relationship, and so those fluids are called Newtonian fluids. Ketchup, though, is part of a merry band of linear rule breakers called Non-Newtonian fluids. Mayonnaise, toothpaste, blood, paint, peanut butter and lots of other fluids respond to force non-linearly. That is, their apparent thickness changes depending on how hard you push, or how long, or how fast. And ketchup is actually Non-Newtonian in two different ways. Way number one: the harder you push, the thinner ketchup seems to get. Below a certain pushing force, ketchup basically behaves like a solid. But once you pass that breaking point, it switches gears and becomes a thousand times thinner than it was before. Sound familiar right? Way number two: if you push with a force below the threshold force eventually, the ketchup will start to flow. In this case, time, not force, is the key to releasing ketchup from its glassy prison. Alright, so, why does ketchup act all weird? Well, it's made from tomatoes, pulverized, smashed, thrashed, utterly destroyed tomatoes. See these tiny particles? This is what remains of tomatoes cells after they go through the ketchup treatment. And the liquid around those particles? That's mostly water and some vinegar, sugar, and spices. When ketchup is just sitting around, the tomato particles are evenly and randomly distributed. Now, let's say you apply a weak force very quickly. The particles bump into each other, but can't get out of each other's way, so the ketchup doesn't flow. Now, let's say you apply a strong force very quickly. That extra force is enough to squish the tomato particles, so maybe instead of little spheres, they get smushed into little ellipses, and boom! Now you have enough space for one group of particles to get passed others and the ketchup flows. Now let's say you apply a very weak force but for a very long time. Turns out, we're not exactly sure what happens in this scenario. One possibility is that the tomato particles near the walls of the container slowly get bumped towards the middle, leaving the soup they were dissolved in, which remember is basically water, near the edges. That water serves as a lubricant betwen the glass bottle and the center plug of ketchup, and so the ketchup flows. Another possibility is that the particles slowly rearrange themselves into lots of small groups, which then flow past each other. Scientists who study fluid flows are still actively researching how ketchup and its merry friends work. Ketchup basically gets thinner the harder you push, but other substances, like oobleck or some natural peanut butters, actually get thicker the harder you push. Others can climb up rotating rods, or continue to pour themselves out of a beeker, once you get them started. From a physics perspective, though, ketchup is one of the more complicated mixtures out there. And as if that weren't enough, the balance of ingredients and the presence of natural thickeners like xanthan gum, which is also found in many fruit drinks and milkshakes, can mean that two different ketchups can behave completely differently. But most will show two telltale properties: sudden thinning at a threshold force, and more gradual thinning after a small force is applied for a long time. And that means you could get ketchup out of the bottle in two ways: either give it a series of long, slow languid shakes making sure you don't ever stop applying force, or you could hit the bottle once very, very hard. What the real pros do is keep the lid on, give the bottle a few short, sharp shakes to wake up all those tomato particles, and then take the lid off and do a nice controlled pour onto their heavenly fries.
Pomfrit je ukusan. Pomfrit sa kečapom je pravi mali raj. Problem je što je u suštini nemoguće sipati tačno odgovarajuću količinu. Toliko smo navikli da sipamo kečap da ni ne primećujemo koliko je čudno njegovo ponašanje. Zamislite bocu za kečap napunjenu čvrstom supstancom, jakom poput čelika. Nikakvim treskanjem nije moguće izvući čelik. Zamislite sada tu istu bocu napunjenu tečnošću poput vode. To bi se izlilo kao san. Ipak, čini se da kečap ne zna šta činiti. Da li je on čvrsta supstanca? Ili tečnost? Odgovor je, pa - zavisi. Većina uobičajenih tečnosti poput vode, ulja i alkohola, reaguje na silu linearno. Ako ih pritisnete dvostruko jače, one se kreću dvostruko brže. Ser Isak Njutn, poznat po anegdoti s jabukom, prvi je uočio ovu relaciju, pa se stoga takve tečnosti nazivaju njutnovskim tečnostima. Kečap, pak, pripada veseloj družini narušitelja ovog linearnog zakona koje se nazivaju ne-njutonskim tečnostima. Majonez, pasta za zube, krv, boja, puter od kikirikija i mnoge druge tečnosti reaguju na silu nelinearno. Drugim rečima, njihova prividna debljina menja se zavisnosno od toga koliko jako pritišćete, koliko dugo ili koliko brzo. A kečap je zapravo ne-njutonovski na dva različita načina. Prvi način: što jače pritisnete, čini se da će kečap izaći tanji. Do određene sile pritiska kečap se, u suštini, ponaša kao čvrsta supstanca. Ali, jednom kada prevaziđete tu kritičnu tačku, kečap menja ponašanje i postaje hiljadu puta tanji nego što je pre bio. Zvuči poznato, zar ne? Drugi način: ako pritisnete silom manjom od granične, kečap će najzad početi da teče. U ovom slučaju, vreme, a ne sila, je ključ za oslobađanje kečapa iz staklenog zatvora. U redu, dakle, zašto se kečap ponaša tako čudno? Pa, pravi se od paradajza, usitnjenog, smrvljenog, drobljenog, potpuno uništenog paradajza. Vidite ove sitne čestice? To je ono što ostaje od ćelija paradajza nakon što prođu kroz tretman kečapa. A tečnosti oko tih čestica? To je pretežno voda i nešto sirćeta, šećera i začina. Kad kečap samo sedi okolo, čestice paradajza su ravnomerno i nasumično raspoređene. Sada, recimo da vrlo brzo primenite slabu silu. Čestice naleću jedna na drugu, ali ne mogu da se mimoiđu, pa time ni kečap ne teče. Pretpostavimo da brzo primenite jaku silu. Ta dodatna sila je dovoljna da iscedi čestice paradajza, pa se možda umesto malih sfera ispretumbaju u male elipse, i bum! Sada imate dovoljno prostora da jedna grupa čestica prođe kraj drugih, pa kečap teče. Recimo sada da primenjujete vrlo slabu silu, ali dovoljno dug vremenski period. Čini se da i nismo baš sigurni šta se dešava u ovom scenariju. Jedna od mogućnosti je da se čestice paradajza blizu zidova posude polako pomeraju ka sredini, ostavljajući supu u kojoj su rastvoreni, a koja je u osnovi voda, blizu ivica. Ta voda služi kao mazivo između staklene boce i središnjeg čepa kečapa i tako kečap teče. Druga mogućnost je da se čestice polako međusobno preuređuju u puno malih grupa koje se zatim međusobno mimoilaze. Naučnici koji proučavaju protok tečnosti i dalje aktivno istražuju na kom principu kečap i njegovi veseli prijatelji funkcionišu. Kečap u suštini postaje tanji što jače pritiskate, ali druge supstance, poput Ubleka, ili nekih prirodnih putera od kikirikija, zapravo postaju deblje što jače pritiskate. Druge se mogu uspeti uz rotirajuće štapove ili nastaviti da se izlivaju iz čaše, nakon što ste ih počeli izlivati. Sa aspekta fizike, kečap predstavlja jednu od komplikovanijih smeša. I kao da to nije dovoljno, balans sastojaka i prisustvo prirodnih zgušnjivača poput ksantanske gume, koji se takođe nalazi u mnogim voćnim napicima i mlečnim šejkovima, može značiti da se dva različita kečapa mogu potpuno drugačije ponašati. Ali većina će pokazati dva karakteristična svojstva: naglo stanjivanje pri graničnoj sili i postepenije stanjivanje nakon što se mala sila primenjuje duži vremenski period. A to znači da kečap možete istisnuti iz boce na dva načina: ili mu date niz dugih, sporih laganih treskanja, pazeći da nikad ne prestanete da primenjujete silu, ili možete jednom snažno udariti bocu. Ono što pravi profesionalci rade jeste da drže poklopac, daju boci nekoliko kratkih, oštrih protresanja, kako bi probudili sve one čestice paradajza, a zatim skinu poklopac i dobro kontrolisano sipaju kečap na svoj nebeski pomfrit.