Should the smooth side of the filter paper be inside or the rough side?

The smooth side faces inside (towards the filtrate)
According to the search results, the smooth side of the filter paper should face the filtrate (i.e., facing inward), and the rough side should face outward (such as sticking to the glass wall or funnel).

1. ‌The role and advantages of the smooth side‌
The smooth side has better wetting properties and can absorb liquid more efficiently, thereby speeding up the filtration speed.
The rough side has a looser structure. If it faces inward, it may cause uneven penetration of the filtrate or poor particle retention.

2. ‌The function of the rough side‌
The rough side facing outward (such as sticking to the funnel) can enhance the fit with the filtration equipment and reduce the risk of filtrate leakage.
Some scenarios (such as diesel particulate collection) require the rough side to face up to intercept particulate matter, but it needs to be adjusted according to the specific experimental purpose.

3. ‌Precautions‌
‌Filter paper type‌: Different materials (such as acid and alkali resistant dragon filter paper or Hofler filter paper) may have different requirements for the front and back sides, and you need to refer to the product description.

‌Filtration speed‌: Too fast flow rate may cause filter paper to break, so pressure needs to be controlled.
‌Summary‌: In conventional filtration operations, it is a common practice to face the smooth side inward, but special applications (such as high particle load) need to be adjusted according to needs.