WASHINGTON, DC. MAY 15, 2013: Adding the liquid nitrogen, while the line of customers grows, Sandra Tran, is a co-owner of Nice Cream Factory a pop-up ice cream shop at Science Club, a bar and restaurant on 19th St in Washington, DC. (Jeffrey MacMillan/JEFFREY MACMILLAN)

Sandra Tran and Gil Welsford might have attracted stares as they unloaded a sink from Tran’s Toyota Forerunner, and placed it on the patio of Science Club, a bar and restaurant in Dupont. The sink was followed by a small fridge, a few standing mixers, two 80-pound jugs of liquid nitrogen, and an assortment of ingredients.

The two entrepreneurs were setting up their fledgling ice cream business, the Nicecream Factory, which uses liquid nitrogen to quickly produce fresh ice cream. Through an agreement with Science Club owner Steve Maguire, the Nicecream Factory will pop up outside the restaurant during off-hours on Wednesday, Thursday and Friday afternoons this summer.

An increasingly common phenomenon in Washington, pop-ups allow entrepreneurs to test out concepts at other, often more established businesses. A nascent coffee business might set up shop every so often in a brick-and-mortar book store, for instance, feeding off an established client base and gathering feedback.

While pop-up agreements may give entrepreneurs an opportunity to refine their products, the logistics of inhabiting another business’s space can proving challenging, as Tran and Welsford have discovered in their first few trials.

As customers lined up on the stairs of Science Club, Welsford and Tran, assisted by a couple of friends, were rapidly taking orders, blending ingredients, and rushing to refill a pitcher of liquid nitrogen from behind a small countertop they’d brought. They make the ice cream by combining sweet cream and other ingredients in a stand mixer — strawberry puree or pistachio butter, for example — and pouring in about half a liter of liquid nitrogen per serving to instantly freeze the mixture. The nitrogen produces a thick white steam as it evaporates almost instantly, and Tran and Welsford are quick to assure customers it’s completely safe to consume — after all, Welsford said, “nitrogen is 78 percent of the air."

They sell small servings for $4.72 plus tax, and large for $5.66.

When the two recent graduates of James Madison University founded the Nicecream Factory a couple months ago, they couldn’t yet afford retail space, which might set them back hundreds of thousands of dollars a year. They opted instead for pop-up arrangements at events, farmers markets, and now, restaurants. They knew most other businesses wouldn’t have the kind of equipment they’d need to make ice cream, so they’d likely have to bring it all with them, and adapt to whatever setting they were in for the day.

Most places, like the Science Club, don’t have an outdoor sink they could use. So with the help of Tran’s father, a computer engineer, they designed and built their own portable outdoor sink — one that could fit in their car — for about $1,200. (They also ran a Kickstarter campaign to crowdfund other equipment costs, raising more than $5,000 within three weeks).

Despite these solutions, any pop-up business is limited by the parameters of their host’s space, Tran explained. “We can only have three or four flavors — we wanted to add more, but if we were to buy two more mixers, we wouldn’t know where to put them,” she said, noting that the Science Club patio couldn’t accommodate more counter space.

Pop-ups can require compromises from the host business too, according to Maguire. He and his staff spend a small amount of time clearing a space for the pop-up, and ensuring a smooth transition in and out of his business. “They’re geographically displacing one table, which could be a six-person table with beers and shots,” he said, but added, “or, it could be a one-person table reading a newspaper for three hours.”

A 17-year veteran of Washington’s restaurant industry, Maguire has hosted several pop-ups at Science Club — clothing retailers, coffee businesses, and jewelry shops — though he first heard of them as “trunk shows.”

He said he has yet to negotiate a revenue split with the Nicecream Factory. But the transient businesses like it have led to a noticeable bump in Science Club’s business, often diversifying his clients, Maguire said. He recently noticed a group of four people walking past the bar, looking for a place to drink, but one of them didn’t want a beer. It was during the Nicecream Factory’s hours, she got an ice cream instead. “Otherwise, they would have gone somewhere else.”