I am currently using a PDMS chip to produce water droplets with a diameter of 200 um in paraffin oil with 3wt%. After a couple of hours, the stationary droplets stored in the chip starts to form a solid layer around them. (Attached photos)
I know the droplets will shrink because of water evaporation, and I try to fix it by changing the base into a thin PDMS membrane with water underneath, but I don't think it's the loss of water that causes it.
The paraffin oil I'm using is from Sigma-Aldrich Product number: 18512-1L, and Span 80 is also from Sigma-Aldrich Product number: S6760-250ML.
Should I change the product I'm using, or are there ways to prevent this from happening.
Hello Ivan，
I dont think that the evaporation of water causes this problem because the diffusion coefficient of water in paraffin is very small.
There is one thing i dont understand in your photos. Why your water droplets form solid after several hours of standing？If u added some polymers inside the water？
Try one thing which might ameliorate your droplets when they are stored on a pdms film：add 1wt sds inside your water phase. And see what happens. Because I saw your droplets are not spherical and have a contact angle less than 180 on the pdms film. Hence，they wet a little bit your pdms film. Adding sds could probably solve it.
Best regards
Thanks for your reply Jiupeng.
I believe water diffused through PDMS then into the air. But the stuffs surrounding the droplets in the third picture seems to be inevitable.
I have searched for some papers which they also use paraffin oil and Span 80, but they didn’t mention about the problem I’m facing.
Hi，that can be true. Can you put the collecting pdms film in a sealed or closed chamber？By this approach，the evaporation of water mignt be slower then a open pdms film in air. Or simplely， u just put a cover on top of the pdms container.
I think probably they also faced this problem but they dont mention it because they have no need to store their droplets for that long time. The best solution is to ask these people about this problem.
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