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Rotationally driven colloidal particles are a particularly nice model system for exploring this pattern formation, as the dominant interaction between the particles is hydrodynamic. Here, we use experiments and large-scale simulations to explore how strong confinement alters dynamics and emergent structure at the particle scale in these driven suspensions. Surprisingly, we find that large-scale density fluctuations (many times the particle size) emerge as a result of confinement, and that these density fluctuations sensitively depend on the degree of confinement. We extract a characteristic length scale for these fluctuations, demonstrating that the simulations quantitatively reproduce the experimental pattern. Moreover, we show that these density fluctuations are a result of the large-scale recirculating flow generated by the rotating particles inside a sealed chamber. This surprising result shows that, even when system boundaries are far away, they can cause qualitative changes to mesoscale structure and ordering.<\/jats:p>","DOI":"10.1063\/5.0310202","type":"journal-article","created":{"date-parts":[[2026,2,12]],"date-time":"2026-02-12T13:00:37Z","timestamp":1770901237000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Emergent clusters in strongly confined systems"],"prefix":"10.1063","volume":"164","author":[{"ORCID":"https:\/\/orcid.org\/0009-0001-5915-2899","authenticated-orcid":false,"given":"Pamud Akalanka","family":"Bethmage","sequence":"first","affiliation":[{"name":"Department of Physics and Astronomy, Northwestern University 1 , Evanston, Illinois 60208,","place":["USA"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0009-0009-5094-5110","authenticated-orcid":false,"given":"Ryker","family":"Fish","sequence":"additional","affiliation":[{"name":"Department of Applied Mathematics and Statistics, Colorado School of Mines 2 , Golden, Colorado 80401,","place":["USA"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6005-1668","authenticated-orcid":false,"given":"Brennan","family":"Sprinkle","sequence":"additional","affiliation":[{"name":"Department of Applied Mathematics and Statistics, Colorado School of Mines 2 , Golden, Colorado 80401,","place":["USA"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0930-9579","authenticated-orcid":false,"given":"Michelle M.","family":"Driscoll","sequence":"additional","affiliation":[{"name":"Department of Physics and Astronomy, Northwestern University 1 , Evanston, Illinois 60208,","place":["USA"]}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"317","published-online":{"date-parts":[[2026,2,12]]},"reference":[{"issue":"3","key":"2026021208003495100_c1","doi-asserted-by":"publisher","first-page":"1143","DOI":"10.1103\/revmodphys.85.1143","article-title":"Hydrodynamics of soft active matter","volume":"85","year":"2013","journal-title":"Rev. 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