What's the trick to seal sous vide foil if liquid contained

I would say that having lots of liquid makes your life easier, not more difficult, because you don't need the vacuum at all.

The purpose of the vacuum is to have heat conduction take place properly. If you were to put a piece of dry food in a plastic bag and close that, the air trapped in the bag would act as an insulator, preventing from the water bath to heat the food.

The good thing about marinades and other liquids used in cooking is that they are great heat conductors. So if you have a bag full of liquid, with pieces of solid food swimming in it, the sous vide process works as intended, without any vacuum.

The most practically convenient way to go about it is to use ziplock bags without a vacuum device. You place your food-with-liquid in the bag, submerge most of the bag, and manually press out the air, then seal by hand. A few air bubbles will be left inside, but they're not a problem.

From there, you can proceed the same way as with a heat-sealed voided bag.

A trick I use is to freeze the liquid into flat pieces and put the frozen marinade in the bags before sealing. Once the liquid thaws you can then squish it around the food in the bag.

The same technique works with non-liquids as well, like butter and pastes. Anything soft tends to get sucked out when using a sealer.

I am assuming you are using a simple consumer vacuum sealer. It is not uncommon for these devices to struggle with sealing a very wet product. Here are a few things that might help, although I cannot promise 100% effectiveness.

• Note that for sous vide cooking, you don't need a perfect vacuum. You merely need to take enough air out of the bag for the product to touch the water (via the plastic) essentially everywhere. Thus, your second 'workaround' is a good approach.
• You will have noticed that when you pull the air out of a sous vide bag, the liquid will not start to flow until (almost) all air has been pulled out. Your first workaround (simply using a bigger bag) gives you more time for that liquid to reach the sealer, and will therefore also work. It is wasteful, however.
• Similarly, being very alert with switching from 'vacuum mode' to 'seal mode' (if your device works like that) might help you avoid getting liquid in the sealer.
• A trick I have found to work quite well is to elevate the vacuum sealer with respect to the bag. Thus, the liquid will be held back by gravity while you pull the vacuum.
• Finally, if your device lets you do this, you might try holding the 'seal mode' for longer. This gives you a chance to boil off/displace liquid that prevents a seal. My sealer comes with 'dry' and 'wet' modes explicitly for this purpose (although I tend to prefer manual operation).

One trick that works very well for sealing liquids with any vacuum sealer: Freeze your liquid before sealing (in whatever container) then transfer to the bag to be sealed. That way you don't get any kind of issues with any machine, since you're now sealing solid food!

We certainly have this problem with our home-grade vacuum sealer, and I usually find a combination of a few things works well.

• Try to limit the liquid to the minimum required for the effect; marinades in a sous vide don't need to be very substantial in most cases, as the vacuum sealing process helps them penetrate, and particularly if it's a long cook time, that longer time at an elevated temperature also helps with marinade penetration.
• For smaller amounts of liquid, use a bit more bag, and hit the 'seal' button before the liquid gets to the top. This will be needed with basically any amount of liquid, only thing that varies is the amount of liquid vs. amount of bag.
• With large amounts of liquid, don't bother sealing the top at all. Put it in a regular bag (or if it's easier, a for-vacuum bag only sealed on one side), and then put it in the sous vide obviously air-side up. As long as the liquid is sufficient to cover the thing to be cooked, the sous vide will raise the temperature of that liquid to the cooking temperature, and that gives you your cooking action. You can use a zip-top bag if you're worried about food safety issues related to the bag being open.
• Some models of Foodsavers, at least, come with a vacuum "sipper" that is basically just a tube with vacuum at one end, and then a glass "marinator" container. This lets you marinate under vacuum in a shorter time (at least in theory, I don't know how well this actually works; studies seem somewhat negative). You could try that, if you want, to avoid marinating during the cooking process. (You don't actually cook in the container, note; this is a pre-cooking step only.)

But I think the main thing is that if you can, marinate before you sous-vide, if you really want a large amount of liquid. If you're willing to give it a day or so before you cook, this will get the marination done without worrying about sealing a zip-top bag.