DIRECT MEASUREMENT OF WEAK DEPLETION FORCE BETWEEN TWO SURFACES

• Published in: Chinese journal of polymer science Vol. 29; no. 1; pp. 1 - 11
• Main Authors: Gong, Xiang-jun, Xing, Xiao-chen, Wei, Xiao-ling, Ngai, To
• Format: Journal Article
• Language: English
• Published: Heidelberg Chinese Chemical Society and Institute of Chemistry, CAS 2011
• Subjects: Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Condensed Matter Physics; Feature Article; Industrial Chemistry/Chemical Engineering; Polymer Sciences; 弱相互作用; 直接测量; 纳米气泡; 聚合物分子; 胶体粒子; 表面相互作用; 高分子表面活性剂; Microgel particles; Nanobubbles; Total internal reflection microscopy (TIRM); Depletion interaction
• ISSN: 0256-7679; 1439-6203
• DOI: 10.1007/s10118-010-1012-8

In a mixture of colloidal particles and polymer molecules, the particles may experience an attractive ＂depletion force＂ if the size of the polymer molecule is larger than the interparticle separation. This is because individual polymer molecules experience less conformational entropy if they stay between the particles than they escape the inter-particle space, which results in an osmotic pressure imbalance inside and outside the gap and leads to interparticle attraction. This depletion force has been the subject of several studies since the 1980s, but the direct measurement of this force is still experimentally challenging as it requires the detection of energy variations of the order of kBT and beyond. We present here our results for applying total internal reflection microscopy （TIRM） to directly measure the interaction between a free-moving particle and a flat surface in solutions consisting of small water-soluble organic molecules or polymeric surfactants. Our results indicate that stable nanobubbles （ca. 150 nm） exist free in the above aqueous solutions. More importantly, the existence of such nanobubbles induces an attraction between the spherical particle and flat surface. Using TIRM, we are able to directly measure such weak interaction with a range up to 100 nm. Furthermore, we demonstrate that by employing thermo~sensitive microgel particles as a depleting agent, we are able to quantitatively measure and reversibly control kBT-scale depletion attraction as function of solution pH.