You change the tone of your voice and your vocabulary when you speak with different people. You do not speak the same way with your mentors, family, friends, or business associates. With family and friends, you typically use a casual, likely light-hearted tone, and your speech is probably sprinkled with slang or abbreviated words. Your speech further changes if you are speaking with a child instead of another adult. You use familiar words to convey your meaning, whether it is a serious message or a joke. However, your conversations and interactions are more formal with your mentors or colleagues. You automatically use a more reserved tone. Overall, the type of relationship you have dictates the words you use with others.
Scientific papers are formal papers, so you should use a formal tone and formal language. Contractions, including didn’t, wasn’t, and won’t, should not be used. These words should be spelled out in full (did not, was not, and will not). “Til” should also be spelled out (until). Other informal words and phrases that should not be used include “a lot of” or “a bit of.” It is better to state a specific amount in a scientific paper than to generalize. Some other vague words that this editor has recently encountered are “bad,” “big,” and “good.” Again, while it is best to be as detailed as possible, you can substitute the words “negative,” “large,” and “useful” or “positive,” respectively, if you absolutely cannot provide a precise quantity.
“Stuff” and “things” are two very informal words that should never appear in a scientific paper. They are simply too vague to be of any help to your readers. Remember, specific is best.