Usage patterns
〈事〉がジワジワ(と)広がるfor something to spread gradually and steadily
The rumor spread slowly but steadily.
〈物〉がジワジワ(と)しみ込むfor a liquid to seep in slowly
The rainwater slowly seeped into the ground.
〈物〉がジワジワ(と)効いてくるfor an effect to gradually take hold
The medicine gradually started to take effect.
汗がジワジワ(と)出るfor sweat to gradually well up
Sweat gradually started beading up from the heat.
〈事〉がジワジワ(と)迫るfor something (a deadline, danger, etc.) to close in gradually
The deadline was steadily closing in.