What Forces Encourage Liquid Down a Straw?

[Eliza452]

Ok, I did not take thermodynamics and my internet searching made little headway but gave me more questions.

When you drink out of a straw - you apply a suction force overcome the force of gravity. The moment that you stop "sucking" - the liquid goes back down the straw.

What is the force that encouraging the liquid to go down the straw?
My first guess was gravity.

I was also wondering if there was a pressure difference. However, if both ends of the straw are open to the atmosphere - they are both at atmospheric pressure so there is no pressure difference to encourage the liquid to go down the straw? Are pressure and gravity related at all depending if you tilted the straw or something?

To minimize too many forces in my head - I'm focusing on a "slug" of water - that was the term used in some of the internet articles.

My last question is that I ran into the concept of surface tension. It sounds like surface tension/capillary action can lift liquid up but wouldn't their be an equal surface tension on the opposite end of the slug?

Can anyone one help me guide me with what terms/definitions I should be looking at?

 

[DaveC426913]

When you drink out of a straw - you apply a suction force overcome the force of gravity.

More accurately, you lower the pressure inside your (rigid) body, causing ambient air pressure in the (let's say juice container) to push the liquid up the staw.

What is the force that encouraging the liquid to go down the straw?
My first guess was gravity.

Yes. Once pressure at both ends of the straw equalizes, gravity pulls the liquid back down, just like anything else.

I was also wondering if there was a pressure difference. However, if both ends of the straw are open to the atmosphere - they are both at atmospheric pressure so there is no pressure difference to encourage the liquid to go down the straw?

Not if the juice container is open to air. If it were sealed (such as some tetra packs) then the pressure inside the juice container may be lower, adding to the pull of gravity.

Are pressure and gravity related at all depending if you tilted the straw or something?

Pressure and gravity won't change, no, but changing the slope of the straw will affect friction and rate of movement.