We monitored the formation and development of a single gas bubble on the surface of a spherical particle of size 1.676 mm under unidirectional freezing and thawing (4.6-5.0 mum/s) and for the first time, quantitatively estimated the force exerted oil this particle by measuring the deformation of an attached elastic stick. The bubble would nucleate and grow oil the particle surface closest to the ice front, while the force curve for a freezing-thawing cycle presented a hysteresis characteristic. This force was much greater than ill the case without a bubble, and hence it dominated the engulfment process ill the present freezing tests. The bubble force increased with increasing bubble size and was shown to be mainly attributable to the elastic force by the deformed bubble shape. Comments were made on the need to incorporate the role of bubbles ill predicting the critical velocity to freeze a suspension with high dissolved gas content. (C) 2004 Elsevier Inc. All rights reserved.