The entropic forces on the self-retracting granular chains, which are confined in channels with different widths, are determined. The time dependence of the length of chain remaining in the channel L_(in)(t) is measured. The entropic force is treated as the only parameter in fitting the solution of the nonlinear equation of motion of L_(in)(t) to the experimental data. The dependence of the entropic force on the width of the confining channel can be expressed as a power-law with an exponent of 1.3, which is consistent with the previous theoretical predictions for the entropy loss due to confinement.
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