Does, "Hi co-worker. What can I do for you?" not work? It usually does the trick for me.

"I've tried ignoring her until she asks an actual question, but she'll just wait" - I don't really see the problem here (except for the fact that some will consider it rude). Why do you care whether or not her question gets answered? She hasn't asked you the question yet, it's not your responsibility to answer what you don't know.

The coworker who insists on pleasantries is likely dealing with social anxiety at some level, knowingly or not. I'd suggest asking this question on IPS.SE, as you might get better responses.

@GOATNine I don't think that is always the case. That is simply a communication style that differs from the OP's communication style.

Related / duplicate on IPS: How do I convince someone formalities between internet friends aren't necessary over a casual, informal text chat?

@LaconicDroid It encourages her to keep opening with pleasantries, and adds another step of back-and-forth/interruptions while I wait for her to respond.

What is the cultural area of the co-worker? This can significantly change the approach due to cultural difference

I want to offer a rational as to why people do this that isn't 'just small talk' etc. I do this because people are often slow at responding or simply busy. If I open with just the question the time-to-reply is increased by the time it takes for me to type the question. Leading with 'hi' allows me to fill the time I would have to wait for a reply with me actually typing the (possibly long) question. It's a very effective optimisation not a pleasantry and this is one of the reason Slack includes the Somebody is typing feature so that you know not to tab away when a question is incoming.

@AdamN why is that an optimisation over just writing the question?

@Dukeling Presumably because it still requires a context switch from OP. In other words, it’s still disruptive and time wasting, even if OP doesn’t respond. This is in fact a very real problem and a common complaint from companies that switched from email over to (mostly) Slack. Given the comments and answers it’s clear that OP failed to convey just how much time this wastes.

You could also just say "I'm good, thanks!" then proceed to the point. Less time wasted than ignoring her and waiting for something to happen.

I sometimes joke with people who do this to me over IM, when they ask "Hi! How are you?" I reply with "Not great if I'm honest" and then watch what happens, it throws them off their little script completely. Every time they'll just ask whatever it was they wanted, proof that this whole back and forth is mostly pointless.

@AdamNaylor that only works if you immediately start typing the question, which doesn't seem to be the case here. (And even then it doesn't really, because you're just pre-disrupting the person you want to ask something. You are sacrificing their time for yours.)

@bodgit: Now that can only possibly work in cultures where that question is expected to always be answered with "Fine." ;)

Set up some kind of automated reply function, which picks a random pleasantry to reply with :)

Could you specify on whether this is just for pleasantries or also for people pinging you to see if you're available, as this answer points out?

are you sure you converse for a few a couple of minutes? I can send a lot of IMs in 2 minutes. Sounds more like you can't even be bothered to respond to one or two messages. I think you know the answer is something along the lines of "I'm good, was there anything you needed?"

@SaggingRufus Yes, it's several minutes. She messages me, I pause what I'm doing and go look at her message, respond, wait anywhere from 30 seconds to several minutes while she responds (during which time I might try to return to my original task or do something else, or simply be interrupted by someone else who sees me waiting on her message), then rinse/repeat until she's finally ready to ask her question.

In my company, saying "Hello" via chat is simply a means of ascertaining if somebody is available; i.e. if there's no response, the person can't chat and we'll have to find some other way to communicate. That's all there's to it; we don't deal in pleasantries like "How are you today".